palaeo moisture evolution in monsoonal central asia during the last 50 000 years abstract the late quaternary climate history of monsoonal central asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka or part of this period wet conditions occurred during middle and late marine isotope stage 3 while the last glacial maximum lgm was characterized by dry climate conditions in the region a stepwise climate amelioration is suggested by the climate records following the lgm several climate signals of this period which were reported from high latitude ice core records are preserved in archives from monsoonal central asia as well during the early holocene high effective moisture was inferred from most records from the area dominated by the indian monsoon e g the tibetan plateau suggesting that holocene optimal climate conditions occurred there during this period in contrast areas which are dominated by the south east asian monsoon se monsoon and the westerlies in north western and north central china mongolia do not uniformly show an early holocene climate optimum for this area optimal conditions prevailed during the mid holocene these apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the holocene during the early holocene strengthened insolation possibly caused an enhanced low level convergence over the tibetan plateau which led to the intensification of the summer monsoon the strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the tibetan plateau the areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity the majority of the palaeoclimatic records suggest reduced effective moisture since the late holocene in the region

monsoonal variability in the northeastern arabian sea during the past 5000years geochemical evidence from laminated sediments abstract a high resolution laminated sediment record from the oxygen minimum zone in the northeastern arabian sea off pakistan was studied to reconstruct climatic variations to infer the monsoon driven moisture history in the northeastern arabian sea during the past 5000 years we have used three types of independant proxies 1 varve thickness 2 stable isotope ratios of planktic foraminifera and 3 inorganic geochemical composition on the basis of these proxies we have subdivided the sedimentary record into five intervals the first time interval from 5000 to 3900 yr bp period i is dominated by summer monsoon activity deduced from ti al zr al k al and sr ca ratios and oxygen isotope data of planktonic foraminifera slightly enhanced winter monsoon conditions can be inferred for the period from 3900 to about 3000 yr bp period ii maximum precipitation during enhanced winter monsoon activity at about 3100 yr bp was followed by an onset of aridification at 3000 yr bp which continued until about 2000 2200 yr bp period iii the textural mineralogical and geochemical data suggest that this aridification was a gradual century scale process rather than an abrupt event comparison to late holocene records in nw and sw india the near east and tropical africa supports our interpretation that period iii from 3000 to 2000 yr bp is characterized by a gradual change in the atmospherical moisture budget over much of this area reduced monsoonal rainfall may be linked to variations of eurasian snow cover as deduced from varve thickness and element to al ratios period iv from 2000 to about 1500 yr bp is characterized by a summer monsoon dominated sequence with higher precipitation than during period iii the 18o record shows an abrupt shift at 2000 yr bp coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern arabian sea present day conditions in the makran area were established at 1500 yr bp period v based on inorganic geochemical data and meteorological records at several stations in the makran we conclude that period v is dominated by precipitation during the winter season here declined rainfall associated with the summer monsoon may be related to a less northward position of the intertropical convergence zone itcz during summer and a stronger influence of winter monsoon linked phenomenona

oxygen and carbon isotopes in modern fresh water ostracod valves assessing vital offsets and autecological effects of interest for palaeoclimate studies abstract the isotopic composition of modern benthic ostracod and mollusc carbonate the oxygen isotope composition of lake water 18ow the carbon isotope composition of dissolved inorganic carbon 13cdic and water temperatures were monitored during a year at different water depths in two lakes ammersee and starnberger see southern germany the aim was to test whether the valves are built in isotopic equilibrium and how seasonal water temperatures are recorded in the shells we find systematic offsets between 18o of valve carbonate compared to 18o expected for a theoretical calcite formed in isotopic equilibrium offsets are higher than 2 for different species of the candona group 1 5 for cytherissa lacustris and around 0 8 for limnocythere inopinata and darwinula stevensoni they seem to be constant for all instars of a species and temperature independent mean water temperatures of the calcification period are readily reflected in the valve 18o if corrected for the offsets 13c of valve carbonate is mainly controlled by 13c of the dissolved carbonate in ambient lake water but seems to be biased by micro habitat effects and vital offsets the study provides information on the seasonality of valve formation thus potentially allowing the quantification of temporal and spatial water temperature gradients from specific 18o records

multiple monsoon controlled breakdown of oxygen minimum conditions during the past 30 000 years documented in laminated sediments off pakistan abstract late holocene laminated sediments from a core transect centred in the oxygen minimum zone omz impinging at the continental slope off pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years high sw monsoon controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon toc and redox sensitive elements ni v as well as by a low diversity high abundance benthic foraminiferal buliminacea association and high abundance of the planktonic species globigerina bulloides indicative of upwelling conditions surface water productivity was strongest during sw monsoon maxima stable omz conditions reflected by laminated sediments were found also during warm interstadial events preboreal bolling allerod and dansgaard oeschger events as well as during peak glacial times 17 22 5 ka all ages in calendar years sediment mass accumulation rates were at a maximum during the preboreal and younger dryas periods due to strong riverine input and mobilisation of fine grained sediment coinciding with rapid deglacial sea level rise whereas eolian input generally decreased from glacial to interglacial times in contrast the occurrence of bioturbated intervals from 7 to 10 5 ka early holocene in the younger dryas 11 7 13 ka from 15 to 17 ka heinrich event 1 and from 22 5 to 25 ka heinrich event 2 suggests completely different conditions of oxygen rich bottom waters extremely low mass and organic carbon accumulation rates a high diversity benthic fauna all indicating lowered surface water productivity during these intervals the omz was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods the abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the north atlantic heinrich events and dust peaks in the tibetan loess records the correlation of the monsoon driven omz variability in the arabian sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale

holocene vegetation and climate of the alashan plateau nw china reconstructed from pollen data a set of 55 recent pollen spectra from the alashan plateau and the qilian mountains qilianshan northwestern china has been analyzed the established relationships between the pollen spectra and modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by eastern juyan palaeolake 41 89 n 101 85 e 892 m a s l several qualitative approaches e g indicator plant species artemisia chenopodiaceae and ephedra fragilis type s l ephedra distachya type pollen ratios and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the eastern juyan pollen record fossil pollen data reveal pronounced environmental changes in the studied area of the alashan plateau between ca 10 700 and 1700 cal yr b p desert taxa as chenopodiaceae nitraria calligonum reaumuria were abundant in the vegetation cover around the study site through the whole record pollen spectra dated to ca 10 700 5400 cal yr b p are characterised by highest values of chenopodiaceae e fragilis type and other desert indicating taxa suggesting rather dry climate most favourable conditions are reconstructed between 5400 and 3900 cal year b p on the basis of relative increase in abundance of artemisia pollen a return to dry conditions occurred at about 3900 cal yr b p the lake finally desiccated after 1700 cal yr b p reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal yr b p correlate well with similar events found in the published records from northern and western china and central mongolia however eastern juyan record does not show the early holocene humidity maximum as suggested by the sedimentary data from monsoon influenced areas of china

climate change and cultural response around 4000 cal yr b p in the western part of chinese loess plateau new data suggest that dramatic environmental change in the western loess plateau of china corresponded with substantial changes in human demography ca 4000 cal yr b p these data demonstrate that a rapid climatic transition from wet to dry led to an ecologically devastated period between 4090 and 3600 cal yr b p the sudden reduction in the number of archaeological sites during this period namely a reduction in the total number of sites and a contraction of the areal distribution of sites points to declining agricultural productivity associated with widespread aridification beginning at 4000 cal yr b p 2005 published by university of washington

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

a general cooling trend on the central tibetan plateau throughout the holocene recorded by the lake zigetang pollen spectra abstract a 741 cm long laminated sediment core covering the last 10 800 years was collected from lake zigetang central tibetan plateau 90 9 e 32 0 n 4560 m a s l and analysed palynologically at 69 horizons biome reconstruction suggests a dominance of temperate steppe vegetation mainly artemisia and poaceae on the central tibetan plateau during the first half of the holocene 10 8 4 4 cal ka bp while alpine steppes with desert elements mainly cyperaceae poaceae chenopodiaceae and characteristic high alpine herb families tend to dominate the second half 4 4 0 cal ka bp the artemisia cyperaceae ratio a semi quantitative measure for summer temperature indicates a general cooling trend throughout the holocene dense temperate steppe vegetation and maximum desert plant withdrawal however indicate that a suitable balance of wet and warm conditions for optimum vegetation growth likely occurred during the middle holocene 7 3 4 4 cal ka bp severe early holocene cold events have been reconstructed for 8 7 8 3 and 7 4 cal ka bp

high resolution holocene environmental changes in the thar desert northwestern india sediments from lunkaransar dry lake in northwestern india reveal regional water table and lake level fluctuations over decades to centuries during the holocene that are attributed to changes in the southwestern indian monsoon rains the lake levels were very shallow and fluctuated often in the early holocene and then rose abruptly around 6300 carbon 14 years before the present 14c yr b p the lake completely desiccated around 4800 14c yr b p the end of this 1500 year wet period coincided with a period of intense dune destabilization the major harrapan indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase

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

a pollen record of holocene climatic changes from the dunde ice cap qinghai tibetan plateau pollen preserved in ice cores from the dunde ice cap provides a sensitive record of holocene climatic changes and vegetational response in the northern qinghai tibetan plateau at time scales ranging from millennia to centuries and decades pollen analysis of the annually resolvable ice layers for a 30 yr period 1957 1986 suggests that total pollen concentration is correlated positively with summer precipitation and negatively with summer temperature thus it is a sensitive indicator of moisture availability and vegetation density in the steppe and desert regions around dunde high pollen concentrations between 10 000 and 4800 yr b p suggest that the summer monsoon probably extended beyond its present limit to reach dunde and westernmost tibet in response to orbital forcing the summer monsoon retreated time transgressively across the qinghai tibetan plateau during the middle holocene relatively humid periods occurred at 2700 2200 1500 800 and 600 80 yr b p probably as a result of neoglacial cooling prominent pollen changes during the medieval warm period 790 620 yr b p and the little ice age 330 80 yr b p suggest that the vegetation in the qinghai tibetan plateau region is sensitive to abrupt century scale climatic changes such as those anticipated in scenarios of greenhouse warming

the holocene salinity history of bosten lake xinjiang china inferred from ostracod species assemblages and shell chemistry possible palaeoclimatic implications abstract a sediment core from bosten lake the largest freshwater lake in the tianshan mountains and in the northwesternmost province of china provided a high resolution record of environmental change covering the last 8500 cal a bp higher salinity levels of bosten lake were reconstructed from assemblages the stable isotope composition and sr ca and mg ca ratios of ostracod shells between 7800 and 5600 cal a bp a period of low and moderate salinities between 5600 and 4900 cal a bp was followed by the re establishment of higher salinities between 4900 and 4300 cal a bp since then bosten lake has recorded fluctuating conditions with a number of multi centennial phases of low and moderate salinities although the salinity lake level relationship of bosten lake is not fully understood a higher salinity at the core site seems to correspond to periods of higher lake levels this contradictory interpretation is explained tentatively by the changing distance of the core site to the freshwater providing river mouth with changing lake levels and potentially resulting lateral shifts of the salinity distribution pattern within the lake assuming this explanation is correct the record from bosten lake reflects higher moisture availability and a strong climatic influence of the indian monsoon in the early mid holocene until about 4000 cal a bp and more unstable drier conditions susceptible to minor changes in the regional circulation patterns and temperature variations afterwards

geochemistry of ostracode calcite part 2 the effects of water chemistry and seasonal temperature variation on candona rawsoni abstract reconstruction of lake paleochemistry from the isotope and trace element composition of fossil ostracodes requires an understanding of geochemical variations induced by the seasonality of shell formation monthly field collections of live candona rawsoni from two hyposaline lakes in the eastern dakotas reveal large variations in 18 o values 2 5 o and mg ca and sr ca ratios up to 35 among individual adults within the same assemblage because the measured variations in water chemistry were small the amplitude of ostracode 18 o and mg ca variations must result from seasonal temperature variation ostracode sr ca ratios which should be temperature independent shows strong positive covariance with shell mg ca these results imply that sr uptake in ostracode calcite increases with the mg content of the shell the partitioning coefficient for mg on the other hand appears to decreases at high values of mg ca in the host water the geochemical variability in the field collections indicates that c rawsoni populations are composed of multiple overlapping generations that attain maturity at different times many juveniles apparently undergo several molts in mid summer and persist as penultimate instars a 1 into the following spring geochemical analysis of late instar juvenile c rawsoni from lacustrine sedimentary records is likely to provide information on mid summer conditions large numbers of fossil ostracodes should be analyzed from each stratigraphic interval to eliminate seasonal noise from the paleochemistry records of temperate region lakes

holocene lake level fluctuations and environmental changes at taro co southwestern tibet based on ostracod inferred water depth reconstruction lake level change is an important indicator of changes in the hydrological status of lakes triggered by the influence of atmospheric circulation systems previous studies show that the interaction

intensified mid holocene asian monsoon recorded in corals from kikai island subtropical northwestern pacific abstract coupled records of sr ca and oxygen isotope ratios 18 o of coral skeletons have been used to produce quantitative estimates of paleo sea surface temperature sst and 18 o of surface seawater that can in some cases be converted to sea surface salinity sss two fossil corals from kikai island in the subtropical northwestern pacific a location affected by east asian summer and winter monsoons were analyzed to investigate differences between mid holocene and present day sst and sss at 6180 cal yr bp ssts were roughly the same as today both in summer and winter 18 o seawater and sss values were higher both in summer 0 5 1 1 psu and in winter 0 2 0 6 psu than modern values at 7010 cal yr bp ssts were slightly cooler both in summer and winter 0 8 and 0 6 c whereas 18 o seawater and sss had higher values in summer 0 3 0 6 psu and in winter 0 8 1 9 psu than present day values these results are consistent with other marine records for the mid holocene of the low and midlatitudes in the northwestern pacific such regional conditions indicate that the east asian summer and winter monsoons were more intense in the mid holocene which was likely a function of the mid holocene insolation regime

correlation between indian ocean summer monsoon and north atlantic climate during the holocene there has been a number of investigations for the correlation between the asia monsoon and the north atlantic climate for the last glacial however little research has been done for the present interglacial the holocene here we present for the first time a high resolution composite proxy record for the indian ocean summer monsoon spanning around 12 000 years based on the 13c time series of both a single plant species carex mulieensis remains cellulose and the total plant assemblage cellulose in the hongyuan peat bog from the tibet plateau the records show that the strength of the indian ocean summer monsoon had abrupt variations during the last 12 000 years the weakest monsoon occurred in the younger dryas period following rapid strengthening from around 11 200 to 10 800 a bp the monsoon kept a generally strong level for around 5300 years from around 5500 a bp onwards the monsoon strength tended to gradual decrease in addition there are a series of abrupt variation events of the monsoon strength on centennial to millennial time scales which superimpose the general tendency of the monsoon variation in every case when the ice rafted debris events in the north atlantic occurred the summer monsoon strength decreased correspondingly these evidences show that teleconnection between the indian ocean summer monsoon and the north atlantic climate is present not only in the last glacial but also in the holocene which may be linked to abrupt reorganizations of the ocean thermohaline circulation leading to redistribution of energy changing temperature and moisture gradient over the southern subtropical indian ocean and eventually controlling the variability of the indian ocean summer monsoon

speleothem evidence for changes in indian summer monsoon precipitation over the last 2300 years speleothems from a well ventilated dolomitic cave in the pokhara valley central nepal preserve a mineralogic record of indian summer monsoon variability over the past 2300 yr annually deposited aragonite layers formed between 2300 and 1500 yr b p indicating reduced monsoon precipitation and increased cave aridity whereas alternating calcite aragonite laminae deposited after 1500 yr b p record elevated summer monsoon precipitation and increased cave humidity dense optically clear calcite layers deposited from 450 5 to 360 20 yr b p 1550 to 1640 a d indicate a less evaporative cave environment and suggest moister and or cooler conditions possibly related to climatic change associated with the onset of the little ice age

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

core formation in the earth and moon new experimental constraints from v cr and mn abstract the mantles of the earth and moon are similarly depleted in v cr and mn relative to the concentrations of these elements in chondritic meteorites the similar depletions have been used as evidence that the moon inherited its mantle from the earth after a giant impact event we have conducted liquid metal liquid silicate partitioning experiments for v cr and mn from 3 to 14 gpa and 1723 to 2573 k to understand the behavior of these elements during planetary core formation our experiments have included systematic studies of the effects of temperature silicate composition metallic s content metallic c content and pressure temperature has a significant effect on the partitioning of v cr mn with all three elements increasing their partitioning into the metallic liquid with increasing temperature in contrast pressure is not observed to affect the partitioning behavior the experimental results show the partitioning of cr and mn are hardly dependent on the silicate composition whereas v partitions more strongly into depolymerized silicate melts the addition of either s or c to the metallic liquid causes increased metal silicate partition coefficients for all three elements parameterizing and applying the experimental data we find that the earth s mantle depletions of v cr and possibly mn can be explained by core formation in a high temperature magma ocean under oxygen fugacity conditions about two log units below the iron wustite buffer though the depletion of mn may be due entirely to its volatility however more oxidizing conditions proposed in recent core formation models for the earth cannot account for any of the depletions additionally because we observe no pressure effect on the partitioning behavior the data do not require the mantle of the moon to be derived from the earth s mantle although this is not ruled out all that is required to create depletions of v cr and mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation such conditions could have existed on the moon forming impactor

groundwater discharge to a gobi desert lake during mid and late holocene dry periods abstract high levels of lake eastern juyanze and a wetter climate in the gobi desert nw china were recorded between 5100 and 4100 cal year bp from ostracod distribution shell geochemistry sedimentology and palynological data after about 4100 cal year bp lake levels decreased and groundwater discharge became dominant source of water during short term regressive events at about 4000 3800 and 3500 cal year bp as suggested by a significant difference in modern river water and groundwater chemistry and the geochemical signal of fossil ostracod shells the overall trend towards lower lake levels culminated in repeated episodes of desiccation of lake eastern juyanze between 3200 and 2900 cal year bp individual regressive events as well as the rapid environmental fluctuations of lake eastern juyanze at about 3000 cal year bp were likely caused by mid to late holocene temperature fluctuations recorded by the dunde ice core yao t thompson l g 1992 trends and features of climatic changes in the past 5000 years recorded by the dunde ice core annals of glaciology 16 21 24 and shifts of the southeastern asian monsoon

holocene environmental changes in bangong co basin western tibet part 4 discussion and conclusions abstract a 12 4 m core taken in lake bangong provides a continuous holocene climatic record we summarize information on changes in stable isotope and radiocarbon balances in the lake hydrobiology and vegetal cover in the catchment deduced from detailed analytical results given in the three preceding papers the bangong record is then compared with the environmental history of the neighbouring lake sumxi also constructed from multidisciplinary analyses the two records show a major environmental change at 10 9 5 ka b p attributed to a rapid strengthening of the summer monsoonal circulation which led to wet warm conditions this event was followed by a long term trend toward aridity which culminated around 4 3 ka b p in western tibet maximum monsoon rainfall seems to have occurred from 9 5 to 8 7 ka b p and from 7 2 to 6 3 ka b p as two wet pulses separated by a reversal event centered on 8 0 7 7 ka b p our results broadly agree with the records from lake seling in central tibet and lake qinghai at the plateau s northeastern margin and with palaeoclimatic studies in western china which document conditions wetter and warmer than those of today during the early middle holocene the environmental fluctuations recorded in western tibet appear in phase with climatic changes recognized in tropical north africa suggesting that the 8 0 7 7 ka b p and the 4 0 3 0 ka b p dry events were caused by abrupt disequilibrium in the climatic system

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

late quaternary climate precipitation δ18o and indian monsoon variations over the tibetan plateau the himalaya tibet orogen contains one of the largest modern topographic and climate gradients on earth proxy data from the region provide a basis for understanding tibetan plateau paleo climate and paleo elevation reconstructions paleo climate model comparisons to proxy data compliment sparsely located data and can improve climate reconstructions this study investigates temporal changes in precipitation temperature and precipitation o18 op18 over the himalaya tibet from the last glacial maximum lgm to present we conduct a series of atmospheric general circulation model gcm echam5 wiso experiments at discrete time slices including a pre industrial pi pre 1850 ad mid holocene mh 6 ka bp and lgm 21 ka bp simulations model predictions are compared with existing proxy records model results show muted climate changes across the plateau during the mh and larger changes occurring during the lgm during the lgm surface temperatures are 2 0 4 0 c lower across the himalaya and tibet and 5 0 c lower at the northwest and northeast edge of the tibetan plateau lgm mean annual precipitation is 200 600 mm yr lower over on the tibetan plateau model and proxy data comparison shows a good agreement for the lgm but large differences for the mh large differences are also present between mh proxy studies near each other the precipitation weighted annual mean 18op lapse rate at the himalaya is about 0 4 km0 4 km larger during the mh and 0 2 km0 2 km smaller during the lgm than during the pi finally rainfall associated with the continental indian monsoon between 70 e 110 e and 10 n 30 n is about 44 less in the lgm than during pi times the lgm monsoon period is about one month shorter than in pi times taken together these results document significant spatial and temporal changes in temperature precipitation and 18op over the last 21 ka these changes are large enough to impact interpretations of proxy data and the intensity of the indian monsoon

aeolian sediments on the north eastern tibetan plateau abstract aeolian sediments on the tibetan plateau are an important archive of palaeoclimatic information this study presents a detailed analysis of sediments from the donggi cona catchment on the north eastern part of the tibetan plateau long and short distance sediment transport leads to a complex pattern of aeolian sediment deposition that depends on climatic changes as well as on the availability of sediments based on the largest dataset of osl datings 51 from a single catchment on the tibetan plateau so far different periods of increased sediment transport have been reconstructed increased aeolian deposition in this high elevation environment started in the early holocene with the accumulation of sands from around 10 5 to 7 ka loess sediments have been preserved from a period from 10 5 to 7 5 ka both archives are related to the strengthening of the asian summer monsoons characterized by wetter and warmer climate this change in climate supported the trapping of aeolian sediments under full monsoon conditions from around 9 ka onwards fluvial processes resulted in erosion of the aeolian archives and the formation of colluvial sediments until 6 ka a dry and cooler climate resulted in the reactivation of dune sands from 3 ka to present possibly in combination with stronger human influence aeolian sediments on the tibetan plateau therefore indicate two different climatic modes during the early holocene wetter conditions were favourable to retain aeolian sediments the reactivation of sediment in the late holocene due to small scale disturbances in the vegetation cover points to a cooler and drier climate

early to mid pleistocene ostracod δ18o and δ13c in the central tibetan plateau implication for indian monsoon change as one of the pioneering projects of the chinese environmental scientific drilling cesd program a 206 5 m long sediment core ce was retrieved from co ngoin co lake basin in the central tibetan plateau limnic records of the abundance of ostracod shells and their oxygen and carbon isotope measurements together with the geochemical and the sedimentological investigations were carried out on the interval between 143 and 52 m core depths corresponding to ca 2 01 0 84 myr distinct oscillations of the abundance and the chemical compositions of ostracod shells have yielded the original information about variations in lakewater chemistry in association with the change in the indian monsoon over the period of 2 01 to 0 84 myr the climate conditions during the early pleistocene were characterized by frequent oscillations and the ostracod records had expressed its synchronous response to these oscillations a major climate change occurred in the co ngoin area at about 0 93 myr a pronounced fluctuation in ostracod delta 18o and delta 13c during the mid pleistocene transition mpt is possibly associated with an increase in continental ice sheets the compositional and the sedimentological characters of the immediately overlying sediments imply a shallow aquatic environment and also a harsh salinity condition beyond the tolerance limits for ostracod survival resulting in the disappearance of the qinghaicypris crassa huang 1979 around 0 84 myr

climate changes of the northeastern tibetan plateau since the late glaciation inferred from clay mineralogy of sediments in kuhai lake a high resolution study of clay mineralogy has been carried out on a sediment core recovered from kuhai lake in order to investigate climatic and environmental changes in the northeastern tibetan plateau since the late glacial period clay mineralogy reveals significant changes in sedimentary sources and hydrolysis conditions in the kuhai lake catchment area before 14 9 cal ka bp the predominance of illite and relatively high amount of kaolinite derived mainly from the reworking of sedimentary rocks imply unstable cold arid conditions and significant glacier cover that prevented from weathering of metamorphic and mafic rocks in the high relief of the catchment between 14 9 and 5 4 cal ka bp an increase in smectite and chlorite contents associated with relatively low values of the illite chemistry index indicate that clay minerals derived from the weathering of metamorphic and volcanic rocks in the highlands of the catchment area such modifications were triggered by a melting of glaciers in the lake s catchment during the bolling allerod interstadial followed by an intensification of monsoon precipitations in the early holocene around 10 0 cal ka bp both processes are responsible for significant physical erosion in the highlands together with an efficient transport of smectite and chlorite from areas of high relief to the lake after 5 4 cal ka bp lower smectite and chlorite proportions associated with a predominance of al rich illite and kaolinite indicate a return to relatively dry conditions and a lowering of physical erosion in the drainage basin comparison between clay mineralogy and pollen records indicates that variations in weathering intensity in the kuhai lake catchment area and changes in its surrounding vegetation are synchronous demonstrating the large potential of clay mineralogical analysis to trace climate change of the tibetan plateau

reduced early holocene moisture availability in the bayan har mountains northeastern tibetan plateau inferred from a multi proxy lake record abstract a late glacial and holocene lake sediment record from the northeastern tibetan plateau was investigated using ostracod and geochemical analysis after lake koucha was established about 16 cal ka bp oligohaline conditions gradually converted to mesohaline conditions about 10 cal ka bp clearly suggesting that the water body was a closed basin lake from about 10 cal ka bp the salinity of the lake rose continuously until hyperhaline levels were reached about 7 5 cal ka bp this increase in salinity was accompanied by a falling lake level extreme rainfall and flooding events in a generally dry climate hit the shallow saline lake around 8 5 cal ka bp causing a unique thin gravel containing layer within the continuous fine grained lake sediments the age control of our record does not allow us to correlate this event exactly to the widely recorded 8 2 cal ka bp event however the recovered sediments from lake koucha provide strong evidence that climate changes around 8 5 cal ka bp had significant environmental implications in northeastern tibet after about 7 3 cal ka bp the available moisture significantly increased causing a continuous salinity decline until the present day freshwater conditions of a through flow lake were established about 6 5 cal ka bp in lake koucha this major shift from a shallow saline to a deeper freshwater lake was probably accompanied by a fundamental change from a macrophyte dominated to a phytoplankton dominated lake a slight reduction in available moisture in comparison to the periods before and afterward was recorded between 4 3 and 2 0 cal ka bp in phase with numerous records from monsoonal asia the reconstructed overall change in moisture availability from dry conditions before about 7 3 cal ka bp to wetter conditions afterwards contradicts most palaeoclimate records from the tibetan plateau and its surroundings which show that wetter conditions in the early holocene were replaced by drier and probably cooler conditions in the mid and late holocene however in the semi arid region of lake koucha with a mean annual precipitation of about 500 mm the insolation driven strengthened monsoon in the early holocene may have resulted in a temperature rise and higher evaporation rates which outweighed the increase in precipitation and therefore led to significantly lower moisture availability our record provides further evidence for a regionally non uniform response of the available moisture evolution to the overall monsoonal signal in the holocene climate history of central asia

magnesium content of non marine ostracod shells a new palaeosalinometer and palaeothermometer abstract ostracods are microcrustaceans with calcitic carapaces each consisting of two valves or shells that grow by moulting and regeneration of sequentially larger valves up to nine times before reaching maturity the mg content of these valves is species dependent and is a function of their growth stage and the temperature salinity mg content and mg ca ratio of their host water newly formed partly calcified valves of the non marine nektic ostracod mytilocypris henricae have high mg contents 100 000 ppm mg in caco3 at individual shell weights of 2 g this relative mg content decreases as calcification proceeds until shell weights exceed 120 g whereupon the mg content is constant and useful as an indicator of the ostracod s environment mg ca ratios in individual ostracod valves from several species of the genera mytilocypris and australocypris from culture experiments and a series of australian lakes indicate that the valve s mg content increases as a function of increasing temperature and salinity within the range 11 25 c and for waters containing 24 2325 ppm mg the molar mg ca ratio in these ostracods fully calcified valves increases by 0 0015 or 370 ppm mg in caco3 per 1 c and per change of 125 ppm mg in solution for most mainland australian lakes a change in water composition of 125 ppm mg corresponds to a change of 3 5 in salinity for a given mg content of a lake the mg ca ratio of the water may vary within the range of common lacustrine values without significantly affecting mg uptake in the ostracod valves these relationships can provide palaeosalinities and or palaeotemperatures for ostracods recovered from continental sediments for mg partitioning an increase in temperature and salinity produce increased mg content in ostracods whereas for 18o fractionation in biogenic calcite an increase in temperature produces a decrease in 18o of the ostracod and an increase in salinity leads to an increase in 18o of the ostracod thus a combination of 18o and mg measurements on non marine ostracods should enable unique resolution of palaeotemperature and palaeosalinity variations

temperature variability and vertical vegetation belt shifts during the last 50 000 yr in the qilian mountains ne margin of the tibetan plateau china abstract a 13 94 m long sediment core collected from a medium sized lake in the qilian mountains ne tibetan plateau china was analysed palynologically at 81 horizons the interpretation of indicator taxa yielded various vertical shifts of the vegetation belts these palaeovegetation results have been checked with lake surface pollen spectra from 8 lakes representing different altitudinal vegetation belts our main findings are the following a short period of the late marine isotope stage 3 around 46 000 yr ago was characterized by interglacial temperature conditions with a tree line above its present day altitude during the lgm the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts indicating that the climate was colder by 4 7 c than today markedly higher temperatures were inferred from higher arboreal pollen frequencies between 13 000 and 7 000 yr ago with a holocene temperature optimum and a maximal picea betula mixed forest expansion between 9 000 and 7000 yr ago when temperatures exceeded the present day conditions by at least 1 2 c alpine steppes and meadows and sub alpine shrub vegetation dominated around the lake since the middle holocene suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods

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

holocene east asian monsoon variation inferred from species assemblage and shell chemistry of the ostracodes from hulun lake inner mongolia a sediment core from hulun lake inner mongolia was analyzed for species assemblages and shell chemistry of ostracodes to investigate changes in the hydrology and climate of the east asian summer monsoon margin during the holocene darwinula stevensoni was abundant ilyocypris spp scarce littoral ostracodes absent and mg ca sr ca and 18o were low 11 100 to 8300 yr ago indicating high lake levels and cool fresh waters darwinula stevensoni declined largely ilyocypris spp throve littoral ostracodes were rare and chemical indicators remained in low values 8300 to 6200 yr ago suggesting that the lake continued high stands but water became warm the lake then contracted and water became cool brackish 6200 to 4300 yr ago littoral ostracodes flourished 4300 to 3350 yr ago marking the lowest lake levels of the entire holocene the lake level recovered and water salinity decreased 3350 to 1900 yr ago from 1900 to 500 yr ago the lake maintained the preceding status albeit lowered stands and increased salinities 1100 to 800 yr ago during the recent 500 yr the lake expanded and water salinity decreased the data imply that the east asian summer monsoon did not intensify until 8300 yr ago and weakened dramatically 4300 to 3350 yr ago

variability in ostracod partition coefficients d sr and d mg implications for lacustrine palaeoenvironmental reconstructions living and sub recent ostracods were collected from four lakes and foul springs located in belgium france and spain characterized by low water mg ca ratios from 0 08 to 0 65 valves of ostracods belonging to cyprideis torosa candona neglecta c marchica and c candida have been extensively analysed for their magnesium and strontium contents with respect to calcium comparisons of these results are made using the same elements from the waters in which the ostracod valves calcified our results offer new and somewhat different interpretations with respect to the uptake of mg and sr in the calcitic ostracod valves these findings include the following 1 the partition coefficient l sr for ostracods belonging to the same genus candona is not a constant the observed variability in d sr values could be due to the rare of biological calcification although more investigations are necessary to assess the influence of this factor it is possible that some taxa grouped under the genus candona belong to a different genus 2 even at the species level cyprideis torosa the ostracod d sr cannot be considered to be constant at low water mg ca ratio 0 65 our d sr value for c torosa is higher than those determined by others for c torosa calcified in evolved marine water 3 the mg ca ratio of the water appears as a major factor influencing d mg for ostracods for low water mg ca values similar to 2 d mg drastically increases this phenomenon is also effective for inorganic calcite implying a non biologic effect that could be linked to mg2 adsorption consequently large variations in me content of fossil ostracod valves from selected lacustrine sequences with expected low water mg ca similar to 2 can be due to small variations in water mg ca c 1998 elsevier science b v all rights reserved

a multi proxy analysis of the holocene humid phase from the united arab emirates and its implications for southeast arabia s neolithic populations an early to mid holocene humid phase has been identified in various arabian geo archives although significant regional heterogeneity has been reported in the onset duration and stability of this period a multi proxy lake and dune record from wahalah in the united arab emirates uae documents significant variations in hydrology biological productivity and landscape stability during the first half of the holocene these data reveal that post last glacial maximum dune emplacement continued into the earliest part of the holocene with the onset of permanent lacustrine sedimentation at the site commencing 8 5 ka cal bp a long term shift towards more arid conditions is inferred between 7 8 and 5 9 ka cal bp with intermittent flooding of the basin and distinct phases of instability throughout the catchment area this transition is linked to the southwards migration of the intertropical convergence zone itcz and associated weakening of monsoon rains a peak in landscape instability is recorded between 5 9 and 5 3 ka cal bp and is marked by a pronounced increase in regional dune emplacement these variations are considered alongside the record of human settlement raising important questions about the interactions between population demographics climate and environment in southeast arabia during the neolithic

ecological development of lake donggi cona north eastern tibetan plateau since the late glacial on basis of organic geochemical proxies and non pollen palynomorphs organic geochemical proxy data from surface sediment samples and a sediment core from lake donggi cona were used to infer environmental changes on the northeastern tibetan plateau spanning the last 18 4 kyr long chain n alkanes dominate the aliphatic hydrocarbon fraction of the sediment extract from most surface sediment samples and the sediment core unsaturated mid chain n alkanes nc23 1 and nc25 1 have high abundances in some samples especially in core samples from the late glacial and early holocene toc contents organic biomarker and non pollen palynomorph concentrations and results from organic petrologic analysis on selected samples suggest three major episodes in the history of lake donggi cona before ca 12 6 cal ka bp samples contain low amounts of organic matter due to cold and arid conditions during the late glacial after 12 6 cal ka bp relatively high contents of toc and concentrations of botryococcus fossils as well as enhanced concentrations of mid chain n alkanes and n alkenes suggest a higher primary and macrophyte productivity than at present this is supported by high contents of palynomorphs derived from higher plants and algae and was possibly triggered by a decrease of salinity and amelioration of climate during the early holocene since 6 8 cal ka bp lake donggi cona has been an oligotrophic freshwater lake proxy data suggest that variations in insolation drive ecological changes in the lake with increased aquatic productivity during the early holocene summer insolation maximum short term drops of toc contents or biomarker concentrations at 9 9 cal ka bp after 8 0 and between 3 5 and 1 7 cal ka bp can possibly be related to relatively cool and dry episodes reported from other sites on the north eastern tibetan plateau which are hypothesized to occur in phase with northern hemisphere cooling events

rheological controls on the geometry of deep faults and the tectonic delamination of the continental crust recent seismic reflection profiles indicate that the deep faults in the continental crust may decouple deep within the crust or at the moho in the contribution it is argued that the decoupling horizons were initially zones across which variations in flow strength occur due to mineralogical differences in the principal rock types these can result in the tectonic delamination of the crust and at the moho calculations reveal marked strength differences across the moho once initial delamination occurs weak planes of mylonite develop along such surfaces and are preferentially reactivated during any subsequent deformation it is postulated that during tectonism the continental crust 1 does not act at depth as a single rigid entity 2 should not be considered as being firmly attached to the upper mantle and 3 is capable of moving independantly of the upper mantle

paleoclimate of southwestern china for the past 50 000 yr inferred from lake sediment records abstract long sediment cores 12 5 and 13 5 m from two lakes in yunnan province were used to infer the paleoclimate of southwest china over the past 50 000 yr during the holocene and marine isotope stage mis 3 bio induced carbonate precipitation and organic matter om production was high suggesting warm temperatures and high primary productivity in contrast sediment inorganic carbon ic and organic carbon oc concentrations were low in last glacial deposits from 38 000 to 12 000 cal yr b p indicating cool temperatures and low productivity the 50 000 yr record has alternating peaks of carbonate and coarse grain 38 m quartz that reflect warm moist interglacial or interstadial conditions alternating with cold dry glacial or stadial conditions respectively spectral analysis of the carbonate and quartz signals reveals power concentrated at periods of 7200 and 8900 cal yr respectively that may reflect a nonlinear climate response to precessional forcing at a time of reduced eccentricity modulation mcintyre and molfino 1996 oxygen isotope values of calcite from yunnan lake cores indicate the summer monsoon was weak during the last glaciation from 50 000 to 12 000 cal yr b p the summer monsoon intensified between 12 000 and 8000 cal yr b p but weakened gradually in response to insolation forcing during the mid to late holocene our results support the overpeck et al 1996 model that posits a weak summer monsoon during the last glaciation that responded nonlinearly to insolation forcing when its intensity was affected by eurasian snow cover and ice sheet extent the summer monsoon intensified and responded linearly to seasonal insolation forcing in the holocene when ice volume diminished

tropical climate instability the last glacial cycle from a qinghai tibetan ice core an ice core record from the guliya ice cap on the qinghai tibetan plateau provides evidence of regional climatic conditions over the last glacial cycle 36 cl data suggest that the deepest 20 meters of the core may be more than 500 000 years old the 18 o change across termination i is 5 4 per mil similar to that in the huascaran peru and polar ice cores three guliya interstadials stages 3 5a and 5c are marked by increases in 18 o values similar to that of the holocene and eemian 124 000 years ago the similarity of this pattern to that of ch 4 records from polar ice cores indicates that global ch 4 levels and the tropical hydrological cycle are linked the late glacial stage record contains numerous 200 year oscillations in 18 o values and in dust nh 4 and no 3 levels

a 700 year macrophyte productivity record inferred from isotopes of macrophyte remains and bulk carbonates in lake koucha northeast qinghai tibetan plateau abstract the isotopic composition of total organic carbon toc in lakes has been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments however the main factors that affect the carbon isotopes of toc may vary in different lake sediment records limiting the ability of organic carbon isotopes to explain biogeochemical and environmental changes especially in lakes with a large number of aquatic plants in this study the 13c values of macrophyte remains and bulk carbonate in a sedimentary core from lake koucha were investigated to evaluate their paleoenvironmental implications we found that the bulk carbonate was dominated by authigenic carbonates formed in the lake and that their 18o and 13c values can be used to reconstruct environmental changes in the lake koucha area the macrophyte productivity derived from the carbon isotopic offset between the macrophyte remains and the bulk carbonate eremains bc in the lake koucha area agree well with total solar irradiance tsi records inferred from 10be and temperature records based on tree rings from the qinghai tibetan plateau although the distribution of macrophytes is related to water depth according past studies and our observations in the field our findings suggest that changes in macrophyte productivity may be primarily driven by temperature changes in the lake koucha area

landscape analysis of the huang he headwaters ne tibetan plateau patterns of glacial and fluvial erosion the large scale geomorphology of the huang he yellow river headwaters centered around the bayan har shan 5267 m asl in the northeastern part of the tibetan plateau is dominated by an uplifted

pollen evidence of changing holocene monsoon climate in sichuan province china pollen evidence from lake shayema mianning county was obtained to examine postglacial vegetation and climatic change in southwestern sichuan china the sclerophyllous character of the region s warm temperate vegetation today is a reflection of extreme drought in spring before the onset of the asian monsoons the pollen record displays several changes in the vegetation over the last 11 000 yr from 11 000 to 9100 yr b p cold tolerant species such as abies betula and deciduous oaks dominated the vegetation between 9100 and 7800 yr b p the abundance of deciduous oaks decreased and evergreen oaks increased as did tsuga and mesic deciduous species this change suggests a warming climate with increased precipitation from 7800 to 4000 yr b p sclerophyllous species increased at the expense of mesic deciduous species an indication that precipitation was becoming more seasonal except for increased disturbance starting ca 1000 yr b p the predominance of sclerophyllous vegetation continued until today the pollen results are compatible with proposed global circulation hypotheses of a strengthened monsoon system during the early to mid holocene

changes in hydrological conditions in the central qing zang tibetan plateau inferred from lake bottom sediments abstract grain particle and chemical analyses of core samples obtained from siling co a lake on the central tibetan plateau and a sedimentation model for the lake with 14 c dating show fluctuations in the hydrological environment of the area during the past 15 000 yr the area of the lake was small and the water level low in the late part of the last glacial period thereafter the lake became larger and the water level high because of meltwater and rainfall this hydrological environment continued until 4000 yr b p at about which time the level of the lake fell sharply and the lake area shrank after a short period of low level and aridity the water level increased markedly but did not reach the previous high level after the large water inflow for a short period the climate became again arid at about 3500 yr b p the change in this interval is closely related to global change statistical analyses of the datasets of the grain particle components of bottom sediments preserved in an aquatic condition over the past 13 000 yr show that most datasets for grain size and chemical parameters had a common period of 2000 2200 yr during this interval

all age depth models are wrong but how badly the uncertainty and error associated with fitted age depth models were investigated by fitting a range of models to sets of simulated radiocarbon dates based on varved sediments and comparing the modelled relationship with the varve timescale with large numbers of dates cubic smooth splines out perform other models with few dates no model reliably provides a good fit to reality and calculated confidence intervals are over optimistic c 2003 elsevier ltd all rights reserved

the sequence of quaternary glaciation in the bayan har mountains four periods of quaternary glaciation in the pleistocene have been found in the area of the bayan har mountains they represent respectively the two stages of the last glacial age the penultimate ice age and an earlier ice age during the late stage of the last glacial age glaciers developed only near the mountain ridges of the bayan har in the earlier stage glaciers expanded down to the piedmont of the bayan har mountains and were as much as 40 km long in the penultimate ice age an extensive ice cover developed depositing an areally widespread till sheet erratics were transported 60 km from bayan har peak to qingshuihe town to the south and to yematan land to the north granitic erratics are also distributed over elevated geomorphological positions such as chalaping ridge the area of ice cover encompassed up to 50 000 km2 furthermore evidence including glacial landforms and sediments shows that a larger glaciation existed in this area in an earlier time possibly during the third most recent ante penultimate glaciation the scale of this glaciated area is greater than those of the more recent events in the qinghai xizang plateau extending over 80 000 km2 this glaciation covered the source area of the yellow river from xingsuhai in the west to duogerangtan in the east including the areas of gyaring and ngoring lakes based on the extent and locations of the glaciations the bayan har stage and galalahai stage of the last glacial age the yematan ice age and the huanghe glacial age can be recognized

quantitative precipitation estimates for the northeastern qinghai tibetan plateau over the last 18 000 years quantitative information regarding the long term variability of precipitation and vegetation during the period covering both the lateglacial and the holocene on the qinghai tibetan plateau qtp is scarce herein we provide new and numerical reconstructions for annual mean precipitation pann and vegetation history over the last 18 000 years using high resolution pollen data from lakes dalianhai and qinghai on the northeastern qtp hitherto five calibration techniques including weighted averaging wa weighted average partial least squares regression wa pls modern analogue technique mat locally weighted weighted averaging regression lwwa and maximum likelihood ml were firstly employed to construct robust inference models and to produce reliable pann estimates on the qtp the biomization method was applied for reconstructing the vegetation dynamics the study area was dominated by steppe and characterized with a highly variable relatively dry climate at 18 000 11 000 cal a b p pann increased since the early holocene obtained a maximum at 8000 3000 cal a b p with coniferous temperate mixed forest as the dominant biome and thereafter declined to present the pann reconstructions are broadly consistent with other proxy based paleoclimatic records from the northeastern qtp and the northern region of monsoonal china the possible mechanisms behind the precipitation changes may be tentatively attributed to the internal feedback processes of higher latitude e g north atlantic and lower latitude e g subtropical monsoon competing climatic regimes which are primarily modulated by solar energy output as the external driving force these findings may provide important insights into understanding the future asian precipitation dynamics under the projected global warming

role of late glacial to mid holocene climate in catchment weathering in the central tibetan plateau the lightness l and concentrations of rb sr and organic carbon corg have been measured in the age constrained lake sediment cores recovered from co ngoin in the central tibetan plateau dissolved sr flux is a dominant control on the variation of rb sr ratios in the sediments variations in color and geochemical proxies of co ngoin sediments display a continuous history of late glacial to mid holocene chemical versus physical weathering intensity in response to past climatic changes between approximately 13 500 and 4500 cal yr b p a lower chemical weathering under a late glacial climate was followed by a higher weathering during the holocene optimum weathering intensity in the central tibetan plateau catchment also responds to well known climatic events such as the younger dryas yd and possibly the holocene event 5 he 5 although there are differences in time or duration of the climatic events many of the well known late glacial to mid holocene events occurred in high elevation co ngoin where atmospheric circulation might play a hemispherical role in climatic forcing the sediment hiatus since c 4200 14c yr b p in the co ngoin indicates a period of desiccation that was probably associated with a sharp decrease in summer monsoon strength our lascustrine results not only imply catchment weathering variations in response to late glacial to mid holocene climatic conditions in the central plateau but also provide further evidence for global connections between regional climates

potential forcings of summer temperature variability of the southeastern tibetan plateau in the past 12 ka abstract investigating potential forcing mechanisms of terrestrial summer temperature changes from the asian summer monsoon influenced area is of importance to better understand the climate variability in these densely populated regions the results of spectral and wavelet analyses of the published chironomid reconstructed mean july temperature data from tiancai lake on the se tibetan plateau are presented the evidence of solar forcing of the summer temperature variability from the site on centennial timescales where key solar periodicities at 855 40 465 40 315 40 and 165 40 year are revealed by using a band pass filter coherent fluctuations were found in the strength of asian summer monsoon northern hemisphere high latitude climate and high elevation mid latitude 26 n terrestrial temperatures with solar sunspot cycles since about 7 6 ka the two abrupt cooling events detected from the tiancai lake record centered at 9 7 and 3 5 ka were examined respectively coupled with the paleoclimate modeling results the early holocene event 9 7 ka is possibly linked to an ocean atmospheric feedback mechanism whereas the latter event 3 5 ka may be more directly related to external forcing

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

lake qinghai china closed basin like levels and the oxygen isotope record for ostracoda since the latest pleistocene abstract in 1985 a sino swiss limnogeological expedition to china s largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern tibet qinghai plateau since the last glacial maximum lake qinghai a perennial lake with an area of 4278 km2 1985 at 3194 m above sea level receives drainage from 30 000 km2 of catchment near the northwest corner of the plateau the 26 5 m deep terminal lake 1985 with brackish alkaline magnesium sulphate rich waters 14 g 1 1 tds has not overflowed since the last glacial but its levels have responded to climatic and hydrologic changes seismic reflection profiles for the sub bottom sediment structures revealed a complex topography with channels relict terraces tectonic tilting and deformation in deposits buried under a 4 6 m packet of mainly flat bedded sediments from which core samples were recovered the lowermost cored material 14 500 yr b p comprises mainly water lain loess deposited in a smaller flood plain lake of up a few metres depth under cold arid climatic conditions ams radiocarbon dating indicates average sedimentation rates for latest pleistocene and holocene deposits in central basin regions of 0 3 0 5 mm yr 1 the 18o values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation evaporation budgets which have controlled the palaeo levels for lake qinghai the precipitation evaporation budget for the lake qinghai region is largely determined by the asian monsoon system the lake qinghai sediment record shows that there was no large influx of glacoer melt waters into the lake basin during the post glacial the regional climate remained arid until at least 12 500 yr b p when an incipient monsoon was possibly initiated river transported silts first reached the basin centre at 11 800 yr b p when the lake depth may have increased to 5 m possibly a monsoonal weakening for several centuries after 10 800 yr b p europe younger dryas caused a further period of aridity at 10 200 yr b p the basin began filling towards modern lake dimensions in response to a permanent strengthening of the asian monsoon substantial lake level rises probably characterised the periods 10 200 9800 9500 8500 8300 7200 yr b p and through perhaps a century at about 6800 yr b p decreasing levels probably characterised the intervening intervals short term lake level fluctuations through the mid holocene climatic optimum were minimal indicating a very stable climatic hydrologic regime an isotopic steady state for the lake waters since 3000 500 yr b p obscures the climatic hydrologic regime dependent 18o gradient but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time three negative 18o excursions for the lake waters may correspond to known glacier advances on the plateau at 3000 2000 and 1500 yr b p during china s neoglacial

paleosalinity from trace metals in fossil ostracodes compared with observational records at devils lake north dakota usa abstract the chemistry of closed basin lakes responds directly to the hydrologic budget through evaporative concentration of dissolved salts and the sediments of these basins contain several excellent fossil and geochemical proxies for past changes in water chemistry and salinity the trace metal content of ostracode shells among the most promising of these paleosalinity methods is further explored in this study of mg and sr partitioning in candona rawsoni a widespread ostracode of the north american great plains the molar distribution coefficients kd mg and kd sr for this species k d mg ca caco 3 0 004 mg 2 ca 2 h 2 o 9 68 10 5 t c k d sr sr ca caco 3 sr 2 ca 2 h 2 oz 0 406 are determined from laboratory experiments in which c rawsoni are cultured under controlled temperatures and salinities our formulation of kd mg contains a correction term for excess mg that is incorporated into the shell during early calcification the accuracy of the distribution coefficients is tested against field collections of live ostracodes from a suite of 12 dakota lakes the kd inferred values for mg2 ca2 h2o and sr2 ca2 h2o are in close accord with the same ratios measured in the lakes a comparison of ostracode reconstructed and historically measured salinities for devils lake north dakota shows good agreement within the salinity range preferred by c rawsoni 1 10 0 although salinity excursions beyond this optimum are poorly represented in the sedimentary record in most lakes salinity is more readily inferred from mg ca h2o than sr ca h2o because sr solubility is a complex function of several carbonate phases as shown for devils lake sr ca h2o may not vary systematically with the salinity of some brines

oxygen isotopic composition of bulk carbonates in recent sediments from lake kuhai nw china and implications for hydroclimatic changes in headwater areas of the yellow river on the northeastern tibetan plateau abstract the water resource issue is one of the most significant problems in the yellow river basin and has received much attention from the public and government because of the dramatically decreased yellow river streamflow in the last several years in this study oxygen isotopic compositions of bulk carbonates from lake kuhai in the headwaters of the yellow river were evaluated as an indirect proxy of past yellow river streamflow in order to place the recent flow reduction in a long term context the results indicated that 18o values of bulk carbonates from core khc14 1 generally vary with changes in the yellow river streamflows related to the precipitation evaporation temperature ratio above the tangnaihai hydrological station over the past 50 years in general the yellow river streamflow in the headwaters area continued to decrease from the early 1980s to the late 1990s because of decreased precipitation and increased temperature then the streamflow increased with enhancing precipitation over the last two decades in addition 18o values of bulk carbonates in the lake kuhai core roughly correlate with the streamflows of the upper reaches of the yellow river recorded by tree ring width over the past 800 years the enriched 18obc values showed that the precipitation evaporation p e ratio or streamflow was very low and the climate might be very dry in the middle of the 1400s ce in the headwaters of the yellow river over the past 800 years our results suggested that changes in the p e ratio or streamflow of the upper reaches of the yellow river were dominated by variations in indian summer monsoon precipitation over a long time scale consistent with other records from the northeastern tibetan plateau

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

paleoclimatic changes on the southern tibetan plateau over the past 19 000 years recorded in lake pumoyum co and their implications for the southwest monsoon evolution abstract the paleoclimatic changes in the southern region on the tibetan plateau tp after the lgm were studied by multi proxy analysis including pollen ca carbonates toc 13 c toc plant macrofossils and sands with sediment cores from lake pumoyum co close to the east end of the himalayas the following paleoclimatic changes characteristic of the southern tp were suggested by comparing with the paleoclimatic records from various regions within the southwest sw monsoon domain on the tp soon after the lgm deglaciation in the southern tp had already started at 18 5 ka bp and more actively increased after ca 16 5 ka bp with the hydrological circulation oscillating in ca 1200 2000 and 4000 year periods an abrupt increase in humidity under the reduced sw monsoon intensity occurred at 15 0 ka bp as part of further deglaciation processes subsequently the climate amelioration in the southern region started from 15 0 ka bp a few centuries earlier than other regions of the tp and then was further intensified by synchronizing with the bolling allored b a event over the b a period the sw monsoon intensity in the southern region almost continuously and sharply increased unlike many regions on the tp the southern region experienced almost no or substantially small impact of the younger dryas yd event and also other major dry cold reversals such as heinrich 1 event the 8 2 kyr event and an abrupt spell occurring between 5 and 4 ka bp the monsoon intensity in the southern region after the yd event started to rapidly increase from 11 4 ka bp and reached a maximum between 10 8 and 10 0 ka bp after the lgm which lagged about 1200 years behind the maximum insolation in the northern hemisphere from ca 9 6 ka bp the gradual decrease in humidity in the southern tp started and continued until 4 2 ka bp it s aridification trend was characterized by three stepwise transition observed at 9 6 7 7 and 5 5 ka bp however the aridification in the southern tp was rather weak and the climatic conditions were sustained fairly humid and comparably stable the humidity between 4 2 and 2 5 ka bp essentially increased despite the weakened sw monsoon intensity at that time whereas from 2 5 ka bp to the present the climatic conditions became most arid and instable during the holocene as a consequence the holocene climate optimum in the southern region lasted from 11 4 ka bp until 2 5 ka bp and was marked by the earliest start and the latest end timings within the sw monsoon domain on the tp the comparison of the start timings from various paleoclimatic records indicated that the first sw monsoon on the tp after the yd event generated at 11 4 ka bp in the southern region subsequently according to the degree of the recovery of positive vegetation albedo feedback system in each region the monsoon probably extended from the southern tp over the entire tp taking 3000 4000 years based on these results the possible key factors and mechanisms responsible for the paleoclimatic changes in the southern region and also the possible processes as to the evolution of the sw monsoon over the tp are discussed

sharp decrease in summer monsoon strength 4000 3500 cal yr b p in the central higher himalaya of india based on pollen evidence from alpine peat age constrained pollen data and magnetic susceptibility of an alpine peat profile from the garhwal higher himalaya display a continuous record of climate and monsoon trends for the past 7800 yr about 7800 cal yr b p dominance of evergreen oak quercus semecarpifolia alder alnus and grasses in the pollen record reflect a cold wet climate with moderate monsoon precipitation from 7800 to 5000 cal yr b p vegetation was progressively dominated by conifers indicating ameliorated climate with a stronger monsoon a warm humid climate with highest monsoon intensity from 6000 4500 cal yr b p represents the mid holocene climatic optimum between 4000 and 3500 cal yr b p the abundance of conifers sharply decreased with the greatest increase in evergreen oak this trend suggests progressive cooling with a decrease in the monsoon to its minimum about 3500 cal yr b p two relatively minor cold dry events at ca 3000 and 2000 cal yr b p marked step wise strengthening of the monsoon until ca 1000 cal yr b p after a cold dry episode that culminated ca 800 cal yr b p the monsoon again strengthened and continued until today a sharp decrease in temperature and rainfall at 4000 3500 cal yr b p represents the weakest monsoon event of the holocene record this cold dry event correlates with proxy data from other localities of the indian subcontinent arabian sea and western tibet

climate change at the 4 2 ka bp termination of the indus valley civilization and holocene south asian monsoon variability 1 planktonic oxygen isotope ratios off the indus delta reveal climate changes with a multi centennial pacing during the last 6 ka with the most prominent change recorded at 4 2 ka bp opposing isotopic trends across the northern arabian sea surface at that time indicate a reduction in indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south asia the 4 2 ka event is coherent with the termination of urban harappan civilization in the indus valley thus drought may have initiated southeastward habitat tracking within the harappan cultural domain the late holocene drought cycles following the 4 2 ka bp event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14c production rates this suggests that solar variability is one fundamental cause behind holocene rainfall changes over south asia

hydrogen isotopic compositions of long chain leaf wax n alkanes in lake qinghai sediments record palaeohydrological variations during the past 12 ka abstract lake qinghai the largest saltwater lake in china is located on the northeastern qinghai tibetan plateau the lake sediments record the climatic variation and response to global climate change on the plateau numerous studies of the lake have been conducted but various opinions remain regarding the palaeoclimatic conditions for the lake since the last glacial maximum especially the palaeohydrological conditions for the northeastern plateau in the early holocene here we discuss the hydrogen isotopic composition d of long chain leaf wax n alkanes for studying the hydrological changes in the lake since the holocene the results show that on the northeastern qinghai tibetan plateau c 31 d values were quite negative avg 202 4 in the early holocene 8 12 ka indicating a strong monsoon and greater precipitation during that period during 8 4 ka c 31 d values gradually became more positive indicating that the monsoon gradually weakened and precipitation gradually decreased the c 31 d values were more positive in the late holocene with a mean value of 186 3 showing that the regional monsoon weakened and the amount of precipitation was low in addition c 31 d values indicated large amounts of precipitation in the early holocene but the c 29 d values during the same period were significantly more positive than c 31 due to a large contribution from aquatic plants therefore the water level of the lake was low in this period which could be related to strong evaporation of the lake water because of the high temperature in the early holocene our study clarifies the changes in the monsoon and precipitation in the northeastern qinghai tibetan plateau and it provides a new estimate of the water level variation in lake qinghai in the early holocene

holocene environmental changes in bangong co basin western tibet part 2 the pollen record abstract a better knowledge of environmental variations in key regions such as the tibetan plateau which plays a fundamental role in the indian monsoon dynamics and thus on the whole northern hemisphere atmospheric circulation is of great importance we present here a detailed paleoenvironmental study based on pollen analyses of a sediment core from lake bangong results on the modern pollen rain are also discussed as they give valuable information for the interpretation of the fossil pollen data the pollen results of the bangong sequence and their paleoclimatic interpretation are integrated into a chronological framework on the basis of palynostratigraphical methods because of dead carbon in the lake system it has been difficult to establish a reliable chronology of the bangong sequence using a geochemical model for correcting measured 14c ages the pollen record from lake bangong compares well with the record from lake sumxi in western tibet lake bangong core provides a continuous holocene pollen record before 9600 yr b p the dominance of desertic elements in the pollen record reflects an arid environment at about 9600 yr b p an abrupt shift to steppe vegetation suggests the onset of moister conditions and from 9500 to 6200 yr b p proxy data indicate conditions warmer and wetter than today proxy data also indicate short term changes during this interval climate instability is suggested by the development of a local palustral flora which peaks at 7700 yr b p and is interpreted as a lake regression in phase with increase in the percentage of littoral and saline water diatom species between 8600 and 7700 yr b p the first humid pulse culminating at 9000 8500 yr b p is thus followed by drier conditions around 8000 7700 yr b p the interval between 7200 and 6500 yr b p regarded as optimal for vegetation growth from the pollen record corresponds to a second humid pulse a trend towards aridity is observed during the following period with dry events centered around 5500 3900 3200 and 700 yr b p

late glacial vegetation and climate oscillations on the southeastern tibetan plateau inferred from the lake naleng pollen profile we present a late glacial pollen record 17 700 to 8500 cal yr bp from a lake naleng sediment core lake naleng is located on the southeastern tibetan plateau 31 10 n 99 75 e 4200 m along the upper tree line variations in the summer monsoon are evident from shifts in vegetation that correspond to late glacial climate trends from other monsoon sensitive regions alpine steppe was recorded between 17 700 and 14 800 cal yr bp indicating low effective moisture at the study site expansion of alpine meadows followed by advances in the position of tree line around lake naleng suggest that climate became warmer and wetter between 14 800 and 12 500 cal yr bp probably representing an enhancement of the asian monsoon climatic cooling and reduced effective moisture are inferred from multivariate analysis and the upward retreat of tree line between 12 500 and 11 700 cal yr bp the timing and nature of these shifts to warm wet and then cold dry climatic conditions suggest that they correspond to the bolling allerod and younger dryas intervals abies betula forests representing warm and moist conditions spread during the early holocene

sedimentology sedimentary petrology and paleoecology of the monsoon driven fluvio lacustrine zhada basin sw tibet abstract in this paper we present sedimentologic petrographic and paleoecologic data from a 810 m thick section of late neogene ca 9 7 to 1 ma fluvio lacustrine deposits in the high elevation zhada basin of sw tibet the tectonic evolution of the zhada basin region located between the himalayan mountains in the s and the transhimalayan mountains in the n is poorly understood but sediment accumulation was probably initiated in response to the middle late miocene uplift of the gurla mandhata and leo pargil domes in the e and w respectively the sedimentary record can be subdivided into five stratigraphic units with distinctly different lithofacies associations la these are from bottom to top a bedload dominated fluvial la unit 1 0 90 m a nearshore lacustrine deltaic la unit 2 90 220 m an offshore lacustrine la unit 3 220 500 m a nearshore lacustrine fan delta la unit 4 500 600 m and a fan delta alluvial fan la unit 5 600 810 m clast and heavy mineral compositions revealed sediment supply from two different provenance areas one from the south and one from the north east based on catchment geology of the himalaya and transhimalaya ostracode faunal assemblages are largely comparable to those of modern tibetan lakes and allow the reconstruction of paleo lake environments including lake level changes and salinity the sedimentary pattern shows strong cyclicity at various scales m 10 s m and 100 m small scale cycles are interpreted as intrinsically controlled depositional processes of deltas lobe switching whereas medium and large scale cycles are most likely forced by climate fluctuations the overall evolution of the sedimentologic record includes a strong and rapid increase in fluvial discharge in the uppermost portion of unit 1 a transition from fluvial to lacustrine deposition during deposition of unit 2 rhythmic variations in the style of lacustrine sedimentation during deposition of unit 3 an increase in the supply of coarse clastics starting with deposition of unit 4 and a transition from lacustrine to alluvial sedimentation during deposition of unit 5 we speculate that the increase in discharge and presumably precipitation leading to the establishment of lacustrine conditions in the lower part of the section unit 2 is largely due to regional summer monsoon intensification during the late miocene and that the transition from lacustrine to alluvial sedimentation in the uppper part of the section reflects the onset of northern hemisphere glaciation and winter monsoon strengthening at 2 6 ma

spatial variability of 14c reservoir effects in tibetan plateau lakes abstract radiocarbon dating of lake sediments is often hampered by the presence of a lake reservoir effect lre also dead carbon or old carbon effect especially in dry and cold regions with a sparse plant cover in the catchment the tibetan plateau became a hotspot of the palaeoenvironmental and climate research community in recent years and the assessment of present day lres is a crucial prerequisite for the establishment of reliable radiocarbon age depth relationships for lake sediment cores this paper examines the spatial variability of lres within individual lakes through a discussion of new data for lake donggi cona and a compilation of previously published data for five additional lakes where lre data are available for different sites lake reservoir effects for lake donggi cona on the northeastern tibetan plateau were determined for shells of aquatic snails collected alive close to the lake s shore the largest determined lre of 20 000 14 c years is significantly larger than previously reported lres from the central part of the lake and larger than any previously published lre for the tibetan plateau relatively low lres in the central regions of lakes higher lres towards the margins and high lres in tributaries and spring waters are apparently a common pattern of tibetan plateau lakes the differences in lres within individual lakes or catchment areas are attributed to the more prolonged exchange of the lake water s dissolved inorganic carbon with the atmospheric co 2 in central lake regions on the one hand and the increasing influence of 14 c free or poor stream and groundwater due to the dissolution of carbonaceous basement rocks towards its margins generally higher lres were recorded in the three tectonically active lake regions of the six examined catchments and it is speculated that rising crustal co 2 further contributes to the lres in these catchments in addition to these observations and inferences elevated 14 c levels of the atmosphere as a result of nuclear bomb testing are often ignored if lres for modern materials are reported by convention relative to the atmospheric 14 c activity of the year 1950 lre data reported in this way represent unrealistic minimum estimates

exploring quaternary paleolake evolution and climate change in the western qaidam basin based on the bulk carbonate geochemistry of lake sediments abstract this study focuses on the concentration of major and trace elements in the diluted acetic acid soluble fraction of sg 1 core lacustrine sediments from the western qaidam basin tibetan plateau in order to explore the potential of applying bulk carbonate analysis to tracing paleolake evolution and climate change the sg 1 core sediments are considered highly suited to the exploration of the paleoenvironmental significance of bulk carbonate geochemistry because most carbonates in the core are authigenic in origin based on a principal component analysis of the elemental composition of diluted acetic acid leachate four major components with distinct characteristics have been extracted accounting for 80 of the total variance the first and most important component is associated with a high positive loading of mn p y sc and rare earth elements ree the second component is characterized by a high positive loading of ca sr and u the third component displays a high negative loading of mg li and b and the fourth component is associated with a relatively high positive loading of fe v and cr after considering the geochemical behavior of these element assemblies during acid leaching we have concluded that these four components contain information indicative of watershed climate conditions carbonate precipitation lake water salinity and iron oxide content respectively reconstructions of watershed climate change and paleolake evolution using the component scores are comparable to those reconstructed from other independent proxy records indicating a long term trend of lake shrinkage and climatic drying since 2 7 ma with abrupt drying events at 2 5 ma 1 8 ma 1 2 ma and 0 6 ma our results suggest that lake carbonates bear more information about watershed climate and lake evolution than expected and that occurrences of some elements in particular trace elements in the diluted acetic acid leachate are not solely and necessarily linked to carbonate minerals

carbon isotope excursions and microfacies changes in marine permian triassic boundary sections in hungary abstract several permian triassic boundary sections occur in various structural units within hungary these sections represent different facies zones of the western palaeotethys margin the gardony core in the ne part of the transdanubian range typically represents the inner ramp while the balvany section in the bukk mountains of northern hungary represents an outer ramp setting the two sections have different patterns for their 13 c values the balvany section shows a continuous change towards more negative 13 c values starting at the first biotic decline followed by a sharp quasi symmetric negative peak at the second decline the appearance of the 13 c peak has no relationship to the lithology and occurs within a shale with low overall carbonate content indicating that the peak is not related to diagenesis or other secondary influences instead the shift and the peak reflect primary processes related to changes in environmental conditions the continuous shift in 13 c values is most probably related to a decrease in bioproductivity whereas the sharp peak can be attributed to an addition of c strongly depleted in 13 c to the ocean atmosphere system the most plausible model is a massive release of methane hydrate the quasi symmetric pattern suggests a rapid warming cooling cycle or physical unroofing of sediments through slope failure and releasing methane hydrate the gardony 1 core shows a continuous negative 13 c shift starting below the p t boundary however the detailed analyses revealed a sharp 13 c peak in the boundary interval just below the major biotic decline although its magnitude doesn t reach that observed in the balvany section based on careful textural examination and high resolution stable isotope microanalyses we suggest that the suppression of the 13 c peak that is common in the oolitic boundary sections is due to combined effects of condensed sedimentation sediment reworking and erosion as well as perhaps diagenesis

radiocarbon dating of pollen by accelerator mass spectrometry radiocarbon dating of bulk sediments has been the standard method for establishing chronologies in the studies of lake sediment cores which have contributed significantly to our knowledge of late quaternary paleo environments these bulk sediment dates are presumed to be direct ageindicators for the speciments e g pollen or macrofossils which are actually being studied however several recent studies have reinforced long standing apprehensions concerning this presumption in this study we demonstrate for the first time the radiocarbon dating of pollen concentrate samples by accelerator mass spectrometry the dates obtained by this method should provide more reliable radiocarbon chronologies for paleo environmental studies than have been obtainable by bulk sediment dating