late cenozoic lacustrine and climatic environments at tule lake northern great basin usa cores of lake sediment to a depth of 334 m in the town of tulelake siskiyou county northern california document the late cenozoic paleolimnologic and paleoclimatic history of the northwestern edge of the great basin the cores have been dated by radiometric tephrochronologic and paleomagnetic analyses lacustrine diatoms are abundant throughout the record and document a nearly continuous paleolimnologic history of the tule lake basin for the last 3 myr during most of this time this basin tule lake was a relatively deep extensive lake except for a drier and cooler interval recorded by fragilaria species about 2 4 ma the pliocene is characterized by a dominance of planktonic aulacoseira solida implying a warm monomictic lake under a climatic regime of low seasonality much of the pleistocene is dominated by stephanodiscus and fragilaria species suggesting a cooler often drier and highly variable climate benthic diatoms typical of alkaline enriched saline waters commonly appear after 1 0 ma and tephrochronology indicates slow deposition and possible hiatuses between about 0 6 and 0 2 ma the chronology of even numbered oxygen isotope stages approximately matches fluctuations in the abundance of fragilaria since 800 ka indicating that glacial periods were expressed as drier environments at tule lake glacial and interglacial environments since 150 ka were distinct from and more variable than those occurring earlier the last full glacial period was very dry but shortly thereafter tule lake became a deep cool lacustrine system indicating a substantial increase in precipitation aulacoseira ambigua characterized the latest glacial and holocene record of tule lake its distribution indicates that warmer and wetter climates began about 15 ka in this part of the great basin diatom concentration fluctuates at 41 000 year intervals between 3 0 and 2 5 ma and at approximately 100 000 year intervals after 1 0 ma in the late pliocene and early pleistocene aulacoseira solida percentages wax and wane in an approximately 400 000 year cycle the possible response of tule lake diatom communities to orbitally induced insolation cycles underscores the importance of this record for the study of late cenozoic paleoclimate change

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

paleoglaciation of the tibetan plateau and surrounding mountains based on exposure ages and ela depression estimates the tibetan plateau holds an ample record of past glaciations and there is an extensive set of glacial deposits dated by exposure dating here a compilation is presented of 10be exposure ages from 485 glacial deposits with 1855 individual samples on the tibetan plateau and ela depression estimates for the glacial deposits based on a simple toe to headwall ratio approach to recalculate the tibetan plateau exposure ages 10be production rates from 24 calibration sites across the world are compiled and recalibrated yielding an updated global reference 10be production rate the recalculated exposure ages from the tibetan plateau glacial deposits are then divided into three groups based on exposure age clustering to discriminate good well clustered from poor scattered deglaciation ages a major part of the glacial deposits have exposure ages affected by prior or incomplete exposure complicating exposure age interpretations the well clustered deglaciation ages are primarily from mountain ranges along the margins of the tibetan plateau with a main peak between 10 and 30 ka indicating glacial advances during the global lgm a large number of deglaciation ages older than 30 ka indicates maximum glaciation predating the lgm but the exposure age scatter generally prohibits accurate definition of the glacial chronology the ela depression estimates scatter significantly but the main part is remarkably low average ela depressions of 337 197 m for the lgm and 494 280 m for the pre lgm indicate restricted glacier expansion

beach ridge plain evidence of a variable late holocene tropical cyclone climate north queensland australia abstract to date most studies of long term tropical cyclone records from beach ridge plains coral shell and sand have suggested that there has been little variation in the intensity of these events over the late holocene this study of a sand beach ridge plain in northeast queensland australia using sedimentary analysis and luminescence chronology suggests there has been considerable variation in both the intensity and frequency of tropical cyclones here since the mid holocene most of the previous beach ridge studies have been of relatively uniform elevation coral shingle shell and sand beach ridges within the plain here at rockingham bay the sediments are composed of coarse grained sand and there is considerable variation in ridge height and the number of ridges emplaced over different time intervals it may be that pure sand ridges provide a more sensitive record of variations in long term tropical cyclone climatology which in turn may contribute to a considerably improved understanding of the behaviour of this natural hazard

late quaternary environmental change in the bonneville basin western usa abstract excavation and analyses of small animal remains from stratified raptor deposits spanning the last 11 5 ka together with collection and analysis of over 60 dated fossil woodrat midden samples spanning the last 50 ka provide a detailed record of changing climate in the eastern great basin during the late pleistocene and holocene sagebrush steppe dominated the northern bonneville basin during the full glacial suggesting that conditions were cold and relatively dry in contrast to the southern basin which was also cold but moister limber pine woodlands dominated 13 11 5 ka indicating increased dryness and summer temperatures 6 7 c cooler than present this drying trend accelerated after 11 5 ka causing lake bonneville to drop rapidly eliminating 11 species of fish from the lake from 11 5 8 2 ka xerophytic sagebrush and shadscale scrub replaced more mesophilic shrubs in a step wise fashion a variety of small mammals and plants indicate the early holocene was 3 c cooler and moister than at present not warmer as suggested by a number of climatic models the diversity of plants and animals changed dramatically after 8 2 ka as many species disappeared from the record some of the upland species returned after 4 ka and great salt lake became fresh enough at 3 4 and 1 2 ka to support populations of utah chub

palaeo water depth estimation for a 600 year record from nam co tibet using an ostracod based transfer function a modern training set of ostracods from the large slightly saline lake nam co on the southeastern tibetan plateau is presented in total 44 samples covering the largest part of the lake and water depth from 2 to 93 m were taken there are eight ostracod species in surface sediments of nam co leucocytherella sinensis leucocythere dorsotuberosa fabaeformiscandona gyirongensis candona xizangensis candona candida ilyocypris cf mongolica eucypris gyirongensis and limnocythere inopinata in order of mean dominance a morphological variant of l dorsotuberosa the forma postilirata is an indicator form of deep water below the thermocline the 36 samples comprise a sufficient number of ostracods for analysis covering water depths between 3 7 and 89 3 m two transfer function models based on weighted average binomial regression wa br and weighted average partial least square regression wa pls using the programs excel and c2 were established to estimate past water depths of nam co transfer functions were applied to ostracod data from two short cores from nam co covering approximately the past 600 years model results show a lake level minimum occurring during the little ice age lia between about 1490 and 1760 ad this minimum corresponds to results from previous stable isotope analysis and indicates a lake level which was about 5 m below the present day level the more exact transfer function model wa pls was set up using c2 and revealed a higher correlation index r o f 0 91 and a lower standard error of 6 7 m however the more selective because of using indicator taxa only wa br model using excel r 0 88 standard error 10 6 m seems to represent palaeo water depths better at least for the studied short cores

palynological evidence for vegetation cycles in a 1 5 million year pollen record from the great salt lake utah usa pollen analysis of the mid lake indian cove well in the great salt lake collected by amoco production co provides a 1 5 ma record of climatic change correlative with the deep sea oxygen isotope record chronologic control for indian cove is provided by the lava creek b 0 60 ma bishop 0 76 ma and huckleberry ridge 2 06 ma volcanic ashes the processing of close interval 3 m ca 8 ka samples is complete for the upper 628 m 1 5 ma of the indian cove well during the last 750 ka interglacial glacial cycles are expressed as the ratio of juniperus ambrosia sarcobatus interglacial vs picea abies and pseudotsuga glacial correlating the peak abundances of this ratio with the odd numbered oxygen isotope stages has refined the time control provided by three volcanic tephra the average pleistocene sedimentation rate is 0 39 m ka 1 huckleberry ridge 2057 ka 797 m based on the refined time scale resulting from correlation with the marine chronology sedimentation reached maximum values during isotope stages 11 9 and was slow between stages 13 and 11 and stages 9 7 development of the bonneville pluvial cycles from 759 to 600 ka is recorded by changes in wetland and aquatic palynomorphs increased pollen concentration above 150 m 310 ka may result from the diversion of the bear river into the great salt lake basin

chronology of sand ridges and the late quaternary evolution of the etosha pan namibia abstract etosha pan situated at the southern border of tropical africa is a vast endorheic plain in namibia s semi arid north the most recent studies agree that the pan was the floor of a former lake with varying water levels here we explored this idea further by investigating the link between lake level change and records of late pleistocene and holocene climate change the varying lake levels were inferred through sediment analysis and optical dating of sand deposits that form ridges parallel to the current shore along the northern and western margins of the pan our results support the view that the sand ridges are shoreline deposits of an evaporitic lake the ridges result from the interplay between intermittent river discharge and riverine sediment supply from the north prevailing north easterly wind and shore parallel waves therefore they are a proxy for former levels of a perennial lake we infer higher levels during the late pleistocene and a drastic drop shortly after 10 ka since around 8 ka etosha pan was covered by a shallow water body this lake water level reconstruction is not in line with the histories of itcz migration and strength of benguela current upwelling we confirm that the linkages between the evolution of the etosha pan and the climate mechanisms driving hydrological changes in subtropical southwest africa are poorly resolved and need further investigation

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

beach ridge development and sediment supply examples from west africa abstract beach ridges form the dominant mode of holocene coastal progradation in west africa they vary from minor accumulations to massively prograded plains over 100 km long variations in the patterns and morphological expression of these sandy deposits record spatial and temporal changes in various environmental parameters among these sediment supply stands out the importance of this factor is illustrated by the morphological patterns and the current sediment dynamics of minor and major beach ridge plains in this region the simplest beach ridges are related to just one or two sources of sand supply from a nearby source in these examples sand has been supplied by the nearshore zone or by eroding cliffs in many areas beach ridge formation ceased as the sediment supply dwindled because of cliff stabilization or establishment of an equilibrium nearshore profile major beach ridge plains in southern sierra leone and in the bight of benin exhibit complex patterns of development related to a bigger sediment supply from nearshore sources and rivers in southern sierra leone the beach ridge plains have been constructed essentially from sands derived from the nearshore zone locally as in the proximal part of sherbro island shallow nearshore shoals encroached upon by the beach ridge front have led to more massive progradation while engendering complex patterns of beach ridge development the sediment supply has been locally supplemented by sands drifting alongshore from the sewa and moa rivers the sediment supply involved in beach ridge development in the bight of benin highlights the role of longshore drift of sands from the volta delta early stages of progradation were characterized by the development of segmented inner beach ridge barriers separated by still infilling coastal re entrants nearshore sources were particularly important in these early stages of beach ridge progradation as coastal re entrants and their estuaries became infilled following the middle holocene eustatic highstand the longshore drift system evolved from a segmented to a unicellular one the resulting conditions of through drift east of the volta delta allowed for longshore transport of sands from this river for the construction of outer beach ridge barriers over a distance of over 250 km while rivers still actively supply sand to the coast current patterns of beach ridge development depend essentially on the coastal sand transport system which determines either the confinement of incoming sand to the vicinity of the very source area itself its regular delivery to the beach ridge front or its through drift to the local downdrift terminus

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

timing of glaciation during the last glacial cycle evaluating the concept of a global last glacial maximum lgm abstract it has long been known that mountain glaciers and continental ice sheets around the globe reached their respective maximum extent at different times during the last glacial cycle often well before the global last glacial maximum lgm c 23 19 ka which is formally defined by peaks in global sea level and marine oxygen isotope records however there is increasing evidence from around the world that it was not only mountain glaciers which were asynchronous with the global lgm but also some regions of the large continental glaciers the barents kara ice sheet in northern eurasia together with a majority of ice masses throughout asia and australasia reached their maximum early in the last glacial cycle a few thousand years before the global lgm period the east antarctic ice sheet also reached its maximum extent several millennia before the global lgm in numerous mountainous regions at high mid and low latitudes across the world glaciers reached their maximum extent before marine isotope stage mis 2 in mis 5 4 and 3 this is in contrast to most sectors of the laurentide ice sheet the cordilleran ice sheet the se sector of the fennoscandinavian ice sheet and the alpine ice sheet in central europe which appear to have reached their maximum close to the global lgm in mis 2 the diachronous maximum extents of both mountain glaciers and continental ice sheets during the last glacial cycle means that the term and acronym last glacial maximum lgm has limited chronostratigraphical meaning when correlating glacial deposits and landforms

oxygen isotopes in lacustrine carbonates of west china revisited implications for post glacial changes in summer monsoon circulation five lakes in the qinghai tibet plateau and north xinjiang have provided oxygen isotope records covering the past 17 11 cal kyr bp the modern lakes are first presented within the major atmospheric circulation and climate patterns of china with the climate related isotope composition of input water equations used to extract hydrological and climatic information from 18o in carbonates are then given after a discussion on potential causes for 18o changes each of the five isotope profiles is interpreted in the light of its specific environmental data the most significant event observed in all records occurred at 12 11 cal kyr bp an abrupt intensification of summer rainfall associated with an extremely rapid 1000 1500 km westward migration of the monsoon fronts the monsoon regime weakened in stages after 8 7 cal kyr bp holocene maximal aridity is recorded at all sites between about 4 5 and 3 5 cal kyr bp long term changes in monsoon strength as inferred from our isotopic interpretation are in good agreement with climate model reconstructions in the tibet qinghai plateau while observed hydrological changes in north xinjiang are not simulated by the models also the non linearity of the holocene monsoon variations cannot be accounted for by orbital forcing

holocene lake level history of the tangra yumco lake system southern central tibetan plateau massive carbonate banks representing ancient lacustrine deposits are exposed in the catchment of the lake tangra yumco southern central tibetan plateau and nearby lake xuru co nine sediment samples were taken below and above these lacustrine deposits to determine periods of changing lake level using optically stimulated luminescence osl applying a multiple aliquot regeneration mar protocol according to facies and stratigraphy samples below the carbonate banks indicate a rising and samples from above a falling lake level the results indicate that the rising lake level of tangra yumco passed the sampling location at 10 5 and 2 1 ka while a falling lake level passed the sampling location at 0 9 ka the rising lake level of xuru co passed the sampling location at 7 9 and 1 7 ka and a falling lake level at 0 5 ka showing a similar trend as at tangra yumco combining these results with recalculated cosmogenic nuclide ages and previously published feldspar luminescence data allows the establishment of

ecosystem and paleohydrological response to quaternary climate change in the bonneville basin utah we report the results of a detailed paleoecological study of the bonneville basin covering the last 280 000 yr our study used fossil ostracodes and a sedimentological record obtained from the august 2000 glad800 drilling operation at great salt lake we analyzed 125 samples taken at 1 m intervals from site 4 gsl00 4 for ostracodes and other paleoecologic and sedimentologic indicators of environmental change multivariate analyses applied to the ostracode data and qualitative analyses of fossil and sedimentological data indicate an alternation between three major environments at the core site over the cored interval 1 shallow saline or hypersaline lakes 2 salt or freshwater marshes and 3 occasional deep freshwater lakes these environmental changes are consistent with shoreline studies of regional lake level fluctuations but provide considerable new detail on both the timing and environmental conditions associated with the various lake phases our age model using 14 c useries tephra and biostratigraphic chronologies allowed us to associate the core s record of regional paleohydrology with the marine oxygen isotope stages of global climate change the core contains continuous records for the last four glacial interglacial sequences salt freshwater marshes were common during the interglacials and deep freshwater conditions correspond with maximum global ice volume in ois 2 and before a maximum in global ice during ois 6 immediately following deep lake phases crashes in lake level from rapid desiccation resulted in the deposition of thick evaporite units our study suggests that the climate of the great salt lake catchment appears to have been drier during ois 6 than during ois 2 we compare our record of environmental change during ois 6 glaciation with other records from the western united states and find that the overall

a 14 000 year record of fire climate and vegetation from the bear river range southeast idaho usa the vegetation and fire history of the bear river range brr southeast idaho has been reconstructed from pollen plant macrofossils and macroscopic charcoal from lacustrine sediments overall the brr record shows independent responses of vegetation and fire regime to climate variation the reconstructions suggest strong seasonal bias from the proxies evaluated with the pollen record most sensitive to insolation driven summer temperature trends and the charcoal based fire record more sensitive to winter snowpack variability together the proxies suggest that the early holocene experienced larger than average snowpacks but very warm summers warmer than modern summer temperatures were maintained through much of the mid holocene but snowpacks decreased dramatically creating the most extreme xeric conditions in the holocene between 7100 and 6000 bp after 6000 bp summers began to show a consistent cooling trend winter precipitation remained low until 4400 bp after which higher than average snowp

paleohydrologic regimes in the southwestern great basin 0 3 2 my ago compared with other long records of gobal climate abstract nine distinct paleohydrologic regimes in the southwestern great basin over the last 3 2 my are recorded by the lacustrine deposits in km 3 a 930 m core from searles lake california these are characterized as being wet intermediate or dry like today excepting the present incomplete regime each lasted 0 12 to 0 76 my major regime changes 0 01 0 13 0 6 and 2 5 my ago appear to coincide with recognized changes in global ice sheet histories as represented by 18 o and other records from marine sediments but comparable changes 0 3 1 0 1 3 and 2 0 my ago do not appear to coincide closely with comparable perturbations in ice sheet histories however all regime boundaries during the last 1 75 my coincide closely in time with changes in sea surface temperatures in the tropical atlantic and many coincide with other deep sea and continental paleoclimatic boundaries the average duration of these paleohydrologic regimes was about 0 4 my standard deviation 0 2 my or less depending on assumptions and it is suggested that the regime boundaries reflect times of change in global sea surface temperatures possibly controlled in part by the earth s 413 000 yr orbital eccentricity cycle during the wettest and driest regimes in the searles lake area lake levels were not sufficiently affected by the 23 000 to 42 000 or 100 000 yr climate cycles related to high latitude ice sheet fluctuations to produce changes in the lacustrine sediment character during intermediate regimes however when lacustrine sedimentation in this area was more sensitive to climate the sediments in km 3 record lake fluctuations with average frequencies near those of the ice sheets this seems to indicate that the high latitude ice sheet fluctuations caused local climatic perturbations but did not dominate the hydrologic component of climate in this area other lacustrine deposits in the southwestern great basin of california and nevada have ages comparable in part to those of the wet to intermediate regimes indicated by km 3 and they may all be products of finite periods when lake expansion alluvial fan growth increased spring discharge and fluvial deposition were promoted in this area by widespread wet climates glacier expansion in the sierra nevada may also have been primarily an expression of and in phase with these wet regimes

later pleistocene holocene climate conditions of qinghai xizhang plateau tibet based on carbon and oxygen stable isotopes of zabuye lake sediments abstract we present a time series of carbon and oxygen stable isotope records of the last 30 000 14c years throughout the last glacial postglacial cycle from western qinghai xizhang tibet plateau a 20 m core drilled in the south basin of zabuye salt lake was analyzed for inorganic and organic carbon and total sulfur contents 13c and 18o values of carbonates our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the last full glacial and the postglacial phase during later pleistocene times 16 2 kyr bp and a rapid positive shift at the transition from pleistocene to holocene 10 6 kyr bp the first shift is marked by the drop of 18ocarb values of about 10 from 2 to 8 and 13ccarb values of about 3 from 5 to 2 the second shift which occurred at the transition from pleistocene to holocene was of similar magnitude but in the opposite direction isotope data combined with total organic and inorganic carbon contents and the lithological composition of the core suggest this lake was an alluvial pre lake environment prior to ca 28 14c kyr bp during ca 28 16 2 14c kyr bp zabuye lake was likely a moderately deep lake with limited outflow the cool and arid glacial climate led the lake level to drop drastically extended residence time overwhelmed the lower temperature and caused a steady increase of 13ccarb and 18ocarb values and total inorganic carbon content in the sediments during ca 16 2 10 6 14c kyr bp this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed a spike of markedly enhanced 13ccarb and 18ocarb is seen at 11 5 kyr bp probably due to the isotopic effects left behind by the short but severe younger dryas yd event after ca 10 6 14c kyr bp zabuye lake probably closed its surface outflow due to strong desiccation and drastic climate warming the early and middle holocene were characterized by unstable climatic conditions with alternating warmer cooler episodes as indicated by the severe fluctuations of total organic carbon 13c and 18o values a hypersaline salt lake environment was finally formed at zabuye after 5 14c kyr bp when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments severe imbalance of inflow outflow resulted in the drastic increase of total sulfur 13ccarb and 18ocarb values and dominance of halite in the lake since ca 3 8 kyr bp to present

ostracods and stable isotopes of a late glacial and holocene lake record from the ne tibetan plateau abstract ostracod assemblage changes and stable isotopes of ostracod shells were used to reconstruct the salinity history of lake donggi cona on the northeastern tibetan plateau in response to the late glacial and holocene climate history brackish and relatively unstable conditions were inferred for the earlier part of the late glacial between 18 9 and 13 4 cal ka bp and for the early to mid holocene 11 9 6 8 cal ka bp a saline lake existed in the intervening period probably representing colder and drier conditions during the greenland stadial 1 younger dryas in north atlantic region freshwater conditions similar to the present stage of lake donggi cona were established for the first time at 6 8 cal ka bp this inference and the implied increase in moisture availability in the latter half of the holocene is in contrast to most climate records from the tibetan plateau which suggest drier and colder conditions however three lake records from the eastern tibetan plateau including our new record show that the holocene insolation driven monsoonal weakening and temperature decrease caused a catchment specific opposite response of the moisture availability in comparison to those at the majority of sites catchments with relatively low precipitation at present experienced apparently a moisture availability increase during the colder second half of the holocene in contrast to the inferred drier conditions at predominantly generally wetter sites the increase in moisture availability at relatively dry sites mainly results from the lowered temperature and evaporation but is probably further controlled by a complex of factors including the amount and timing of precipitation the altitudes of the lake basins and upper reaches and the existence or development of glaciers snow fields and frozen ground in a given catchment our record from lake donggi cona suggests that the recently observed reduction in yellow river discharge in response to present global warming will likely continue or even increase under warmer climatic conditions in the future

ubiquity of anomalous fading in k feldspars and the measurement and correction for it in optical dating anomalous fading rates were measured in k feldspars separated from 49 sediment samples mainly from north america the intensity of the optically stimulated luminescence was found to decrease linea

a lake level chronology based on feldspar luminescence dating of beach ridges at tangra yum co southern tibet many lakes on the tibetan plateau exhibit strandplains with a series of beach ridges extending high above the current lake levels these beach ridges mark former lake highstands and therefore dating their formation allows the reconstruction of lake level histories and environmental changes in this study we establish a lake level chronology of tangra yum co fifth largest lake on the tibetan plateau based on luminescence dating of feldspar from 17 beach ridge samples the samples were collected from two strandplains southeast and north of the lake and range in elevation from the current shore to 140 m above the present lake using a modified post infrared irsl protocol at 170 c we successfully minimised the anomalous fading in the feldspar irsl signal and obtained reliable dating results the luminescence ages indicate three different stages of lake level decline during the holocene 1 a phase of rapid decline 50 m from 6 4 to 4 5 ka 2 a period of slow decline between 4 5 and 2 0 ka 20 m and 3 a fast decline by 70 m between 2 ka and today our findings suggest a link between a decrease in monsoonal activity and lake level decline since the early holocene

the influence of 14c reservoir age on interpretation of paleolimnological records from the tibetan plateau there is a great deal of controversy regarding the fate of glaciers and ice fields on the tibetan plateau in the face of continued anthropogenic global warming paleoclimate reconstructions and spatial analyses aimed at mapping past climate changes are the key to understanding the climatic response of the tibetan plateau to changing conditions specifically the numerous lakes distributed across the tibetan plateau can provide high resolution spatial and temporal climate reconstructions to investigate past changes in the climate system in this paper we review the primary limitation to exploiting these valuable paleoclimate archives errors in radiocarbon based age models we review the techniques that have been used to estimate 14 c reservoir ages on the tibetan plateau and compile the published 14 c reservoir ages to examine their spatial and temporal patterns and to assess the imposed chronological uncertainties using site specific evaluations of bangong co and lake qinghai we demonstrate that 14 c age model uncertainties permit equally probable and contrasting interpretations of existing paleoclimate records we also examine 14 c induced uncertainties in the spatial climatic response on the tibetan plateau to 1 the termination of the last glacial maximum and 2 the holocene thermal maximum we conclude with recommendations for reducing uncertainties in future lake based paleoclimate studies on the tibetan plateau

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

shrinking lakes in tibet linked to the weakening asian monsoon in the past 8 2 ka expansion or shrinkage of closed lakes is a natural response to fluctuations in precipitation and evaporation linked closely to changes in strength or position of atmospheric circulation in tibet there are many such lakes with paleo shorelines that can be used for reconstructions of climate history despite the fact that many paleo shorelines are well preserved in tibet dating them has been seriously hindered by various difficulties here we present the first optical dating chronology for a series of paleo shorelines in zhari namco the third largest inland lake in central tibet our results indicate that the lake level has dropped 128m over the past 8 2ka younger shorelines are found at lower altitudes indicating that the shorelines follow a geomorphic chronological order and a broadly continuous trend of stepwise shrinkage the surface area of zhari namco has shrunk in size from 4605km2 at 8 2ka ago to 996km2 at present 300km3 of water has been lost from this lake such a loss in water implies a significant reduction in precipitation over the past 8 2ka a likely result of a weakening asian monsoon following the decreasing precipitation since the early holocene this area has become increasingly arid

luminescence dating of beach ridges for characterizing multi decadal to centennial deltaic shoreline changes during late holocene mekong river delta abstract empirical understanding of decadal to centennial scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries we tested the effectiveness of quartz optically stimulated luminescence osl dating of beach ridges to constrain shoreline changes of the mekong river delta over the last 3500 years forty seven osl ages have been analyzed from the tra vinh delta plain central mekong river delta the tra vinh beach ridges are recurved and branching showing the hierarchy of shoreline changes which include discontinuous shifts c 5 km seawards caused by the emergence and elongation of a delta front bar island and subsequent downdrift accretion of spits the spit accretion is interrupted by changes of shoreline orientation each of the discontinuous shifts resulted in a cluster of beach ridges which is referred to as ridgeset the beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c 5 the osl chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges the chronology clearly illustrates the coastal progradation from 3500 years relative to ad 2010 onwards two main types of beach ridge are present recurved and trunk ridges osl ages of recurved ridges are consistently younger both downdrift and seawards documenting decadal to centennial scale shoreline migration especially over the last 1500 years trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset representing the extensive erosion and resedimentation of the updrift coast major changes in shoreline orientation occurred around 400 500 years suggesting strengthening of the northeasterly winter monsoon at the beginning of the little ice age the rate of progradation of the tra vinh delta plain is described in two ways frequency of the discontinuous shift and growth rate of the delta plain area both suggest the coast has expanded regularly over the last 1000 years a decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred possibly affecting the behavior of the modern and future shorelines which can be compared with the less human influenced centennial to decadal scale past shoreline changes reconstructed in this study

revisiting growth and decline of late quaternary mega lakes across the south central tibetan plateau abstract lake basins record critical interactions between the hydrosphere cryosphere and atmosphere their sediments and shoreline landforms archive timing duration and intensity of past climatic variability and environmental impacts over a variety of timescales thousands of lake systems spread across the internally drained tibetan plateau today of which many are only the salty remains of much more expansive paleolakes in the past this study presents new shoreline ages integrated with quantitative digital topographic analysis and regional geomorphic evidence to reconstruct the sizes and extents of late quaternary rift lake systems for the south central tibetan plateau this study presents optically stimulated luminescence osl and the first k feldspar post infrared infrared pirir stimulated luminescence ages for zabuye caka and dawa tso and tests if these lakes were once part of a singular pleistocene mega lake established across four n s rift systems our new results show that two large but separate paleolake systems developed in the latest pleistocene earliest holocene and no lake larger than 6460 km2 has existed since at least 41 ka in the lunggar region early holocene paleolakes expanded up to 7 modern 220 335 km3 with two to four fold asymmetric lake expansion in the west compared to the east findings corroborate earlier investigations implying that dramatic runoff increase and or evapotranspiration reduction prompted rapid earliest holocene lake expansion precipitation variability in combination with permafrost degradation temperature and wind strength governed paleolake moisture balance open system lake behavior and evolving drainage configurations across complex topography explain delayed onsets of lake regression and punctuated lake decline this suggests abrupt holocene climate shifts are not necessarily required for punctuated lake decline lastly unique geomorphological and paleoclimatic similarities between tibetan and east african rift lakes highlight the potential for some tibetan paleolakes to function as natural climate amplifiers over both short and geologic timescales

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

late quaternary water depth changes in hala lake northeastern tibetan plateau derived from ostracod assemblages and sediment properties in multiple sediment records abstract late pleistocene and holocene climate dynamics along the marginal belt of the east asian summer monsoon in china and their responses to hydrological cycles in lake basins of the tibetan plateau are still a matter of scientific discussion hala lake a closed 65 m deep lake basin in the western qilian mountains qinghai province is considered a monitor of climate driven hydrological and environmental changes during the past 24 kyr bp the distribution patterns of ostracod assemblages stable isotopes sediment geochemical properties in four sediment records from different water depths and their combination with the unique limnological setting enabled us to reconstruct four major phases of centennial scale water depth fluctuations from the global last glacial maximum ca 24 kyr bp to the present our results show that hala lake experienced a very shallow and small water body during the lgm and lateglacial under cold and dry climate conditions rapid increase of water depth and contemporaneous lake expansion started at around 14 kyr bp phase i most likely as a result of glacier melt due to the onset of climate warming the lake reached 45 m water depth at around 13 5 kyr bp reduced water depth during the younger dryas spell ca 12 kyr bp may be attributed to a short term return to cooler and drier conditions during the early holocene phase ii water depth increased further toward lake highstands close to its present level with a highest lake level of up to 9 m above the present lakestand at 8 0 7 8 kyr bp besides continued glacier melt supply we assume that summer monsoon effective moisture contributed to the overall water budget but remained relatively unstable favoring water depth fluctuations a pronounced lower water depth falls into the period between 9 2 and 8 1 kyr bp perhaps the result of weak monsoon influence or its complete absence although the warming trend continued toward its optimum at ca 8 7 kyr bp a distinct mass flow most likely triggered by an earthquake occurred during a lake lowstand either at ca 7 0 kyr bp or at around 8 1 kyr bp the mid holocene phase iii was characterized by fluctuating water depths between 7 8 and 4 5 kyr bp conflicting trends of stable isotope data limit the validity of water depth estimations but may show higher lake levels between 5 5 and 4 5 kyr bp coincident with dated lake sediments in a cliff position at the northern lake shore this positive water balance can most likely be attributed to increased westerly derived moisture supply during autumn and late winter although summer monsoon influence could also be of significance coincident with the 4 2 ka event the lake experienced shallow water at around 4 1 kyr bp perhaps as a result of continued cooling and drier climate conditions supporting the arguments of a general cooling trend throughout the holocene the late holocene phase iv is characterized by extremely unstable hydrological conditions with rapid fluctuations in water depth more frequently controlled by westerly driven effective moisture supply since the lake lowstand at about 1 4 1 2 kyr bp the lake has developed toward its present level our research underlines the necessity for comparing multiple proxy records from different lake sites to better evaluate centennial scale climate driven variations throughout the late pleistocene and holocene periods all presented data suggest the variable influence of summer monsoon effective moisture on the hydrological budget of the lake water depth variations did not follow the long term pattern of the asian monsoon system due to a potential modulation by westerly derived moisture impact

high elevation paleoenvironmental change during mis 6 4 in the central rockies of colorado as determined from pollen analysis abstract paleoecological studies from rocky mountain usa high elevations encompassing the previous interglacial mis 5e are rare the 10 m composite profile from the ziegler reservoir fossil site 2705 m asl of central colorado allows us to determine paleoenvironments from marine oxygen isotope stages mis 6 4 using pollen zones that are approximately equivalent to marine oxygen isotope stages during pollen zone pz 6 time pollen assemblages dominated by artemisia sagebrush suggest that alpine tundra or steppe occurred nearby the transition to pz 5e was characterized by a rapid increase in tree pollen initially picea spruce and pinus pine but also quercus oak and pseudotsuga menziesii douglas fir non arboreal pollen nap types increased during pz 5d while abies fir and juniperus juniper increased during pz 5c pollen evidence suggests that temperatures during pz 5b were as cold as during pz 6 with the site again surrounded by alpine tundra picea dominated during pz 5a before the onset of cooler conditions during pz 4 the mis 6 mis 5e transition here was similar to the mis 2 mis 1 transition at other rocky mountain sites however the ziegler reservoir pollen record contains evidence suggesting unexpected climatic trends at this site including a warmer than expected mis 5d and cooler than expected mis 5b

middle and late wisconsin paleobotanic and paleoclimatic records from the southern colorado plateau usa the colorado plateau is a distinct physiographic province in western north america which presently straddles the transition between summer wet and summer dry climatic regimes to the south and northwest respectively in addition to climate the diversity of environments and plant communities on the colorado plateau has resulted from extreme topographic diversity desert lowlands as low as 360 m elevation are surrounded by forested plateaus and even higher peaks greater than 3800 m elevation this environmental diversity provides a unique opportunity to study the history of biotic communities in an arid region of north america although the colorado plateau harbours numerous potential sites the paleoecological record of the plateau is poorly known potential deposits for analysis include packrat middens alluvial and cave sites at lower elevations and lake bog and wetland sites at higher elevations forty six sites have been analysed across the nearly 337 000 km2 region of which 27 contain records that span marine oxygen isotope stage is 2 data with is 3 information coming from only 12 sites most is 2 and 3 sites are clustered along the lowland regions of the colorado river corridor and the uplands of the mogollon rim area we compiled selected data from long paleoecological records to examine patterns of vegetation and climate change across the southern colorado plateau for the middle and late wisconsin during the middle wisconsin mixed conifers covered middle elevations presently dominated by ponderosa pine pinus ponderosa and juniper juniperus woodland grew at elevations today covered by blackbrush coleogyne and sagebrush artemisia desert during the late wisconsin boreal conifers primarily engelmann spruce picea engelmannii replaced the mixed conifer association estimates of mean annual temperatures mat during is 3 were at least 3 4 c cooler than today whereas is 2 mat estimates are at least 5 c colder our investigation of millennial scale climatic variability within the region provided equivocal results the packrat midden sequence could not distinguish vegetation changes that might be associated with heinrich events in the north atlantic from the lake records however many heinrich events were associated with generally drier intervals often with elevated sagebrush pollen concentrations future paleoecological investigations should concentrate on the northern colorado plateau as well as the eastern and western margins additional sites along with closer spaced sampling in regions already studied will be important in determining the history of important climatic phenomena such as the timing of the arizona monsoon

constraining holocene lake level highstands on the tibetan plateau by 10be exposure dating a case study at tangra yumco southern tibet abstract many lakes on the tibetan plateau are surrounded by palaeo shorelines that document former lake level highstands and hence past changes in climate establishing accurate chronologies for these shorelines is crucial to correlate lake level fluctuations with palaeo environmental histories that are commonly inferred from lake sedimentary records here we apply surface exposure dating to constrain the age of two lake level highstands at one of the largest tibetan lakes tangra yumco which has well developed palaeo shorelines located up to 185 m above the current lake for sampling we focussed on wave cut bedrock terraces that vary considerably in width because they were carved into rocks of different erodibility samples from the highest strath terrace 180 185 m above the lake at three different sites yield tightly clustered 10 be ages between 7 39 0 19 ka and 7 87 0 27 ka internal errors which constrain the end of an early holocene lake level highstand at 7 6 0 6 ka external error the consistency of the 10 be ages indicates that bedrock erosion by wave attack reduced the inherited 10 be component to negligible amounts because otherwise samples would show considerable age scatter two bedrock samples from a lower terrace 140 145 m above the lake and one amalgamated clast sample from a beach ridge on the same terrace yield 10 be ages between 4 04 0 14 ka and 4 50 0 15 ka these ages document that the lake level dropped by 40 m between 7 6 ka and 4 3 ka granite boulders from the two investigated terraces yield apparent 10 be ages that are 6 ka and 27 ka older than the ages of the respective terraces indicating that this sample type is not reliable in this setting overall our results demonstrate that exposure dating of bedrock terraces provides a valuable tool to reconstruct lake level histories on the tibetan plateau

long term east west asymmetry in monsoon rainfall on the tibetan plateau variations in the strength and duration of the asian monsoon affect more than half the population of the planet and there is intense interest in where and how much rainfall amounts will change regionally in response to warmer climate conditions the modern monsoon region is divided into distinct subsystems so the response may be regionally heterogeneous northern hemisphere monsoon systems intensified during the early to mid holocene increasing summer rainfall and creating wetter conditions across much of eastern asia in this study we use the area encompassed by high shorelines of early holocene paleolakes in tibet to reconstruct paleorainfall and hence paleomonsoon patterns during this very wet period we found that the early holocene relative paleolake expansions of 130 closed basin lake systems in the central tibetan plateau display a strong east west gradient paleolake areas expanded by approximately fourfold in the western plateau compared to approximately twofold expansion in the eastern region this early holocene pattern mirrors the modern west east climate division on the plateau rainfall in the west is closely tied to the indian summer monsoon ism subsystem and in the east is closely tied to a probable mix of ism and east asian summer monsoon easm subsystems our results suggest that these modern climate divisions are an enduring feature of the plateau and that ism rainfall increased much more than easm rainfall in response to the same insolation forcing

holocene monsoon climate documented by oxygen and carbon isotopes from lake sediments and peat bogs in china a review and synthesis there has been much recent debate about holocene climate variation in the monsoon region of china especially the temporal pattern of variations in precipitation the time transgressive nature of the holocene precipitation maximum and the extent to which variations in regions influenced by the indian summer monsoon ism and the east asian summer monsoon easm have been synchronous we summarize and compare carbonate oxygen isotope records delta o 18 carb from ten lakes within the present day ism region we discuss their paleoclimate significance considering the present day moisture source isotopic composition of precipitation and the hydrological setting the delta o 18 carb records are controlled mainly by the isotopic composition of lake water which in turn is a function of regional precipitation evaporation p e balance and the proportion of precipitation that is monsoon derived we normalized the delta o 18 carb data and used these records to generate an integrated moisture index this index along with oxygen isotope records from speleothems and carbon isotope records delta c 13 org from peats within the monsoon region suggests that holocene climate was broadly synchronous across the monsoon region and within the limits of accuracy of the existing age models provides no strong evidence for previously proposed anti phasing of the ism and the easm stable isotope records from lake sediments and peat bogs have excellent potential for providing high quality paleoclimate data for monsoon asia and complement high resolution speleothem sequences which are only found in certain localities c 2011 elsevier ltd all rights reserved

lake level variations of qinghai lake in northeastern qinghai tibetan plateau since 3 7 ka based on osl dating abstract qinghai lake is the largest internally drained lake in china and its unique location makes it sensitive to climate changes late glacial climate changes associated with variation in qinghai lake levels have been intensively investigated for the past 40 years with particular attention paid to lake level fluctuation histories between the last interglacial and the holocene however the details of lake level fluctuations during the holocene are still unclear using both optically stimulated luminescence osl dating of quartz for 22 samples and infra red stimulated luminescence irsl of feldspars only for sample hyw1 whose irsl age is 37 15 years a total of 23 samples are dated from paleoshoreline deposits fluvial sediments and aeolian sands a total of 13 sections near the modern lake shore with ages from 37 15 to 3710 350 years these ages are used to reconstruct the lake level fluctuation history spanning the last 3700 years the results indicate that 1 the youngest irsl age of 37 15 years is in agreement with the independent age of 39 29 years suggesting that luminescence dating is able to date decadal samples for sediments from the study area 2 the lake experienced several oscillations imposed on an overall regressive trend during the past 3700 years 3 the dated paleoshoreline deposits are generally related to warm and wet periods suggesting that those shoreline deposits formed during cold and dry periods if any may have been modified by later transgressions 4 lake level fluctuations during the period of 3700 240 years ago are generally consistent with the climate conditions identified in other proxies with the highest lake level occurring about 1770 years ago and 5 after 240 years ago the lake level dropped more rapidly which is inconsistent with the proxy records showing a warmer and wetter phase

amino acid paleothermometry of quaternary ostracodes from the bonneville basin utah abstract unlike many other proxy climate recorders temperature dependent chemical reactions that take place in fossils are independent of paleoprecipitation changes and therefore allow paleotemperatures to be calculated uniquely in this study i use the extent of amino acid racemization in fossil ostracode shells to reconstruct the temperature history of the bonneville basin utah the enantiomeric composition d l ratios of 64 ostracode samples each comprising an average of 18 subsamples from 25 sites were analyzed by reverse phase hplc the effective diagenetic temperature edt for five intervals of the quaternary including the last full glacial period was calculated for two amino acids aspartic and glutamic acids errors associated with the edt calculations were derived using a monte carlo procedure age control was provided by both new and previously published ams 14 c luminescence tephra and amino acid geochronology the edt for the southern bonneville basin during the middle pleistocene interval from 620 150 ka oxygen isotope stages ois 16 6 was 5 7 1 5 c this compares with an edt of 10 9 1 3 c for the late holocene last 5 8 cal ka and a current mean annual temperature of about 9 4 c at nearby sites 40 n latitude the edt then decreased to 3 3 1 3 c for the interval from ois 6 2 150 12 cal ka during the last full glacial period 24 12 cal ka ois 2 the edt was 1 1 2 5 c or about 10 3 c lower than during the late holocene paleotemperatures this low suggest that reduced evaporation rather than increased precipitation could have accounted for the increased effective moisture and the growth of lake bonneville

great salt lake and precursors utah the last 30 000 years sediment cores up to 6 5 m in length from the south arm of great salt lake utah have been correlated radiocarbon ages and volcanic tephra layers indicate a record of greater than 30 000 years a variety of approaches have been employed to collect data used in stratigraphic correlation and lake elevation interpretation these include acoustic stratigraphy sedimentologic analyses mineralogy geochemistry major element c o and s isotopes and organics paleontology and pollen the results indicate that prior to 32 000 year b p an ephemeral saline lake playa system was present in the basin the perennial lake which has occupied the basin since this time rose in a series of three major steps the freshest water conditions and presumably highest altitude was reached at about 17 000 year b p the lake remained fresh for a brief period followed by a rapid increase in salinity and sharp lowering in elevation to levels below that of the present great salt lake the lake remained at low elevations and divided at times into a north and south basin until about 8 000 year b p since that time with the exception of two short rises to about 1290 m the lake level has remained near the present elevation of 1280 m

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

holocene variations in the asian monsoon inferred from the geochemistry of lake sediments in central tibet we present a record of monsoon variations for the early and middle holocene that is inferred from the geochemistry of sediment cores from ahung co a lake in central tibet the resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal which eliminates errors associated with the lake hard water effect we made down core geochemical measurements of carbonate organic carbon c n and 13c of bulk organic matter 13c and 18o of carbonate and dolomite proxy calibration and modern water balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation we find that lake level and monsoon precipitation have been decreasing at ahung co since the early holocene 7500 cal yr b p superimposed on this trend are rapid declines in monsoon rainfall at 7000 7500 and 4700 cal yr b p and seven century scale wet dry oscillations the cores do not contain sediment from the last 4000 yr surface sediments from the lake accumulated during the 20th century however from this we argue that lake levels have risen again recently following a late holocene dry period

late glacial and holocene development of lake donggi cona north eastern tibetan plateau inferred from sedimentological analysis abstract sediments of lake donggi cona on the northeastern tibetan plateau were studied to infer changes in the lacustrine depositional environment related to climatic and non climatic changes during the last 19 kyr the lake today fills a 30 8 km big and 95 m deep tectonic basin associated with the kunlun fault the study was conducted on a sediment core transect through the lake basin in order to gain a complete picture of spatio temporal environmental change the recovered sediments are partly finely laminated and are composed of calcareous muds with variable amounts of carbonate micrite organic matter detrital silt and clay on the basis of sedimentological geochemical and mineralogical data up to five lithological units lu can be distinguished that document distinct stages in the development of the lake system the onset of the lowermost lu with lacustrine muds above basal sands indicates that lake level was at least 39 m below the present level and started to rise after 19 ka possibly in response to regional deglaciation at this time the lacustrine environment was characterized by detrital sediment influx and the deposition of siliciclastic sediment in two sediment cores upward grain size coarsening documents a lake level fall after 13 cal ka bp possibly associated with the late glacial younger dryas stadial from 11 5 to 4 3 cal ka bp grain size fining in sediment cores from the profundal coring sites and the onset of lacustrine deposition at a litoral core site 2 m water depth in a recent marginal bay of donggi cona document lake level rise during the early to mid holocene to at least modern level in addition high biological productivity and pronounced precipitation of carbonate micrites are consistent with warm and moist climate conditions related to an enhanced influence of summer monsoon at 4 3 cal ka bp the lake system shifted from an aragonite to a calcite dominated system indicating a change towards a fully open hydrological lake system the younger clay rich sediments are moreover non laminated and lack any diagenetic sulphides pointing to fully ventilated conditions and the prevailing absence of lake stratification this turning point in lake history could imply either a threshold response to insolation forced climate cooling or a response to a non climatic trigger such as an erosional event or a tectonic pulse that induced a strong earthquake which is difficult to decide from our data base

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

reconstruction of holocene coastal foredune progradation using luminescence dating an example from the świna barrier southern baltic sea nw poland abstract two sandy spits on the wina barrier wolin and uznam provide a very detailed succession of holocene coastal foredunes and dunes and are regarded as key sites along the southern baltic sea coast optically stimulated luminescence osl dating is proven to be a powerful tool for the reconstruction of holocene coastal spit evolution and foredune accretion quartz extracted from the coastal sediments in the study area provides excellent properties for osl dating the osl age from the innermost dune indicates that spit development of the wina barrier started immediately after rapid sea level rise of the littorina transgression decelerated at 6 6 ka a significant change in the foredune progradation rate occurred during the late subatlantic transgression at 1 2 ka 800 ad when migration rates decreased from 2 6 0 7 m a 1 to 1 3 0 4 m a 1 progradation accelerated again during the little ice age between 1550 and 1850 ad the systematic dating of 28 samples reveals six hiatuses during foredune succession at 2100 bc 900 bc 200 bc 200 ad 600 ad 1000 ad and 1600 ad the timing of these hiatuses correlates with the phases of transgressive dune development in the surrounding area troszyn and wina and with phases of increased aeolian activity in other parts of north and west europe we conclude that four of these phases of foredune erosion instability and aeolian sediment mobilisation were caused by general climate shifts to cooler and stormier conditions at 2200 bc 900 bc 600 ad and at 1600 ad the latter corresponding to the little ice age the period of increased aeolian activity around 1000 ad is probably to a phase of intensive forest clearance in central europe in contrast the systematic foredune accretion and foredune plain growth correlates to periods of positive sediment budget milder and calmer climate and an intact vegetation cover

beach ridges definitions and significance abstract beach ridges frequent components of quaternary coastal plains and other coastal landforms have been cited as indicators of the positions of ancient seashores and associated sea levels numerous authors utilized the term beach ridge for active and relict usually wave built supratidal and or intertidal forms wind built ridges have been only occasionally included in the definition the term was applied also to submerged landward shifting eventually stranded bars a consistent redefinition of the term is highly desirable beach ridges should include all relict strandplain ridges whether dominated by wave swash built or by eolian lithosomes all active ridge like shore features regardless of dimensions morphology and origin are excluded because of the resistance of coarse clastic ridges to wave and wind erosion swash built gravel or coarse shell storm ridges may build several meters above the level of high tide swash built high berms even on pure sandy beaches exceed the highest tides during episodes of wind induced record water levels frequently but not always burying underlying low relief berm ridges of berm lithosomes sequences of relatively steep multiple foredunes are commonly named beach ridge plains the narrow subparallel relict foredunes that form these strandplains presently are designated as eolian beach ridges beach ridges thus are defined as relict semiparallel multiple wave and wind built landforms that originated in the inter and supratidal zones until separated from the shoreline by progradation sandy pebbly or shell enriched backshore berm ridges behind an active foreshore should not be considered beach ridges strandplain progradation is either continuous or with the inclusion of subtidal cat s eye ponds discontinuous contrary to claims transgressive cheniers do not represent true cheniers alone within their overall progradational context cheniers a special category of beach ridges bracketed by subtidal intertidal mudflats may be transgressive or regressive in character landward driven transgressive ridges should be designated beach ridges only after they are stabilized on intertidal flats when recognizable between clearly identifiable intertidal and overlying eolian intervals the horizontal interface between these lithosomes in beach ridges may help the reconstruction of ancient tide lake levels diagnostic sedimentary textures structures and fossils however often may be unavailable in the deposits along with various types of elevated terraces composed of raised marine deposits and certain coastal landforms of erosional origin that occur worldwide beach ridges of clearly proven wave built origin may also serve as indicators of ancient higher than present sea levels

holocene vegetation and climate histories in the eastern tibetan plateau controls by insolation driven temperature or monsoon derived precipitation changes abstract the climates on the eastern tibetan plateau are strongly influenced by direct insolation heating as well as monsoon derived precipitation change however the moisture and temperature influences on regional vegetation and climate have not been well documented in paleoclimate studies here we present a well dated and high resolution loss on ignition peat property and fossil pollen record over the last 10 000 years from a sedge dominated fen peatland in the central zoige basin on the eastern tibetan plateau and discuss its ecological and climatic interpretations lithology results indicate that organic matter content is high at 60 80 between 10 and 3 ka 1 ka 1000 cal yr bp and shows large magnitude fluctuations in the last 3000 years ash free bulk density as a proxy of peat decomposition and peatland surface moisture conditions oscillates around a mean value of 0 1 g cm 3 with low values at 6 5 4 7 ka reflecting a wet interval and an increasing trend from 4 7 to 2 ka suggesting a drying trend the time averaged mean carbon accumulation rates are 30 6 gc m 2 yr for the last 10 000 years higher than that from many northern peatlands tree pollen mainly from picea mostly reflecting temperature change in this alpine meadow forest ecotonal region has variable values from 3 to 34 during the early holocene reaches the peak value during the mid holocene at 6 5 ka and then decreases until 2 ka the combined peat property and pollen data indicate that a warm and wet climate prevailed in the mid holocene 6 5 4 7 ka representing a monsoon maximum or optimum climate for the region the timing is consistent with recent paleo monsoon records from southern china and with the idea that the interplays of summer insolation and other extratropical large scale boundary conditions including sea surface temperature and sea level change control regional climate the cooling and drying trend since the mid holocene likely reflects the decrease in insolation heating and weakening of summer monsoons regional synthesis of five pollen records along a south north transect indicates that this climate pattern can be recognized all across the eastern tibetan plateau the peatland and vegetation changes in the late holocene suggest complex and dramatic responses of these lowland and upland ecosystems to changes in temperature and moisture conditions and human activities

isolating the role of mesoscale eddies in mixing of a passive tracer in an eddy resolving model 1 this study examines the role of mesoscale eddies in distribution of chlorofluorocarbon cfc in a numerical model of the north atlantic our main conclusions are based on a comparison of two eddy resolving experiments one with and one without eddy advection of cfc the cfc distributions in these two experiments exhibit substantial differences within the subpolar gyre in particular in the presence of eddies the horizontal distribution of cfc in the upper 1000 m is more uniform and the cfc concentration is lower in the lower 1500 m our analysis demonstrates that these basin wide differences are caused by the interactions of eddy advection with convection and mean advection in several isolated locations the corresponding eddy flux divergence terms are spatially non uniform in magnitude their effects are often nonlocal and sometimes work against the spreading of cfc being carried out by the mean advection these conclusions are further confirmed by a third experiment in which the eddy advection was replaced by a down gradient diffusive parameterization our results demonstrate that advection by mesoscale eddies is crucial to the mixing of atmospheric gasses into the ocean and outline difficulties in representing eddies by the down gradient diffusion

luminescence dating of the pasado core 5022 1d from laguna potrok aike argentina using irsl signals from feldspar in this paper we develop and test a luminescence chronology for the pasado core 5022 1d taken in the laguna potrok aike maar lake because of unsuitable quartz osl characteristics sand sized k feldspar extracts were chosen as a dosimeter and the dose was measured using a recently developed post ir irsl pirir290 measurement protocol using this approach we were able to access a stable signal and thus avoid the ubiquitous problem of signal instability in feldspar dating extensive laboratory tests show that the chosen pirir290 protocol is applicable to these samples we also develop a new criterion based on the known relative bleaching rates of the conventional irsl signal ir50 and the pirir290 signals and the relationship between the resulting equivalent doses this is used to identify and reject poorly bleached samples fourteen samples out of 35 were rejected based on this criterion the resulting age depth profile is self consistent increases smoothly with depth and is in agreement with the independent age control reclus mt burney and hudson tephras at the top of the core our new luminescence chronology suggests the 5022 1d core reaches back to 65 ka at 96 m below lake floor

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr