Climatic significance of δ18O records from precipitation on the western Tibetan Plateau

Analysis of daily precipitation samples for stable oxygen isotopes （δ^18O） collected at the Shiquanhe and Gerze （Gaize, Gertse） stations in the Ngari （Ali） region on the western Tibetan Plateau indicates that air temperature affects the δ^18O variations in precipitation at these stations. In summer, Shiquanhe and Gerze show strongly similar trends in precipitation δ^18O, especially in simultaneous precipitation events. Moreover, both stations experienced low δ^18O values in precipitation during the active monsoon period, resulting from the southwest monsoon （the summer phase of the Indian monsoon）. However, during the break monsoon period （during the summer rainy season, when the monsoon circulation is disrupted）, δ^18O values in summer precipitation remain relatively high and local moisture recycling generally controls the moisture sources. Air temperature correlations with δ^18O strengthen during the non-monsoon period （January--June, and October--December） due to continental air masses and the westerlies. In addition, evaporation also influences the δ^18O variations in precipitation. The observed temporal and spatial variations of δ^18O in precipitation on the western Tibetan Plateau and adjacent regions show that the late May and early June-the late August and early September time frame provides an important period for the transportation of moisture from various sources on the Tibetan Plateau, and that the region of the West Kunlun-Tanggula Ranges acts as a significant climatic divide on the Plateau, perhaps for all of western China.     

Synoptic-scale variation of δ18O in summer monsoon rainfall at Lijiang, China

Based on the daily δ~(18)O data in June―September 2003 at Lijiang and the daily mean NCEP/ NCAR reanalysis data, synoptic-scale variation of δ~(18)O in summer monsoon rainfall was investigated. The 'precipitation amount effect' is obvious for the daily δ~(18)O variation, whereas the 'temperature effect' is insignificant. Alternate occurrences of active phase and break phase of the southwest monsoon probably influence the synoptic-scale δ~(18)O variation prominently. Moreover, the isotopic composition in precipitation during the late monsoon months is presumably influenced significantly by recycling of monsoon precipitation. Both the above factors disturb the 'amount effect' of isotopic variation in the monsoon region. This study also indicates that the synoptic-scale rainfall δ~(18)O variation at Lijiang in summer is domi-nated by the Indian monsoon depression (low pressure) system at large scale. These results are important for further studying the 'amount effect' and reconstructing paleoclimate in the monsoon region.     

Oxygen isotope variation in the water cycle of the Yamzho lake Basin in southern Tibetan Plateau

Given the potential use of stable isotope in the paleoclimate reconstruction from lacustrine records as well as in the local hydrology cycle, it is crucial to understand the processes of stable isotope evolution in catchment in the Tibetan Plateau region. Here we present a detailed study on the water oxygen isotope based on 2 years observation including precipitation, river water and lake water in the Yamzho Lake, south of the Tibetan Plateau. Temporal variation of local precipitation 5180 shows an apparent ＂monsoon cycle＂. In monsoon season, 5180 in waters is lower. In non-monsoon season, δ^18O in precipitation and lake water is higher and higher river δ^18O exists in spring, probably reflecting the effect of land surface evaporation, together with the higher δ^18O values in spring precipitation. It is also found that the surface lake water δ^18O varies seasonally and annually. The lower lake water δ^18O in the late summer is apparently related to the summer monsoon precipitation. The mean δ^18O value of lake water in 2007 is 1.2‰ higher than that in 2004, probably due to the less monsoon precipitation in summer of 2007, as can be confirmed from the precipitation data at the Langkazi meteorological data. It is also found that an obvious shift of vertical lake water δ^18O reflects the fast mixture of lake water. δ^18O values of lake water are over 10‰ higher than those of precipitation and river water in this region due to the evaporation fractionation. The modeled results show that the evaporation process of the lake water is sensitive to relative humidity, and the present lake water δ^18O reflects a relative humidity of 51% in the Yamzho Lake. It shows that the lake will take 30.5 years to reach present lake water δ^18O given a large shift in the input water δ^18O. The modeled results also reveal that surface lake water temperature and inflow δ^18O have slight effect on the isotopic balance process of lake water in the Yamzho Lake.     

Indian monsoon influences altitude effect of δ~(18)O in precipitation/river water on the Tibetan Plateau

The δ^18O variation in precipitation acquired from 28 stations within the network of Tibetan Observation and Research Platform （TORP） is studied, with the focus on the altitude effect of δ^18O in river water during monsoon precipitation in an effort to understand the monsoon influence on isotopic composition in annual river water. It is found that δ^18O in precipitation on the Plateau is influenced by different moisture sources, with significant Indian monsoon influence on δ^18O composition in plateau precipitation and river water. The δ^18O of water bodies in the monsoon domain is generally more depleted than that in the westerly domain, suggesting gradual rainout of southwesterly borne marine moisture in the course of long-distance transportation and lifting over the Himalayas. The lapse rate of δ^18O in river water with altitude is the largest during monsoon precipitation, due to the increased temperature vertical gradient over the southern Plateau region controlled by monsoon circulation. The combination of δ^18O in river water in monsoon （wet） and non-monsoon （dry） seasons shows a larger lapse rate than that in non-monsoon （dry） season alone. As the altitude effect of δ^18O in precipitation and river water on the Tibetan Plateau results from the combined effect of monsoon moisture supply and westerly moisture supply, the δ^18O composition and its altitude effect on the Plateau during monsoon seasons should be considered in the reconstruction of paleoelevation of the Tibetan Plateau.     

Tracing the major source area of the mountainous runoff generation of the Heihe River in northwest China using stable isotope technique

Based on the data of δ^18O in precipitation during the period of April 2006 through May 2007 in the upper catchment of the main stream of the Heihe River in the Qilian Mountains, we found that there existed an evident altitude effect on δ^18O in precipitation, and the δ^18O-altitude gradient was -0.18‰/100 m. When δ^18O in river water at the outlet of the mountainous drainage area of the main stream of the Heihe River was combined with the δ^18O-altitude relation, it was realized that the mountainous runoff was generated mostly at high altitudes. Using two component models, we revealed that 80.2% of the annual total mountainous runoff amount was generated at the alpine permafrost-snow-ice zone with an altitude of above 3600 m a.s.I.     

Southwest monsoon changes indicated by oxygen isotope of ostracode shells from sediments in Qinghai Lake since the late Glacial

The δ 18O records of ostracode shells in sediments of core QH-2000 from Qinghai Lake can be used as a better proxy to reflect monsoon changes. Low monsoon precipitation between 17.5 and 11 cal. ka BP is indicated by positive δ 18O values averaging 2.37‰. A fast shift in δ 18O from positive at 11 cal. ka BP to negative at 10 cal. ka BP indicates sharp increase of monsoon precipitation. An interval of generally high monsoon precipitation is observed between 10 and 6 cal. ka BP with δ 18O values averaging -2.15‰. Decrease of monsoon precipitation between 6 and 2.5 cal. ka BP is indicated by positive δ 18O values. δ 18O keeps positive values averaging 3.0‰ between 2.5 and 0 cal. ka BP suggesting low high monsoon precipitation. The climatic changes indicated by δ 18O records of ostracode shells in sedi- ments of core QH-2000 from Qinghai Lake and our broader regional comparison show that the climate in Qinghai Lake since the late Glacial is probably controlled by southwest monsoon other than southeast monsoon.     

The δ18O variation of a stalagmite from Qixing Cave, Guizhou Province and indicated climate change during the Holocene

A stalagmite from Qixing Cave, Guizhou Province was dated with the TIMS U-series method and its oxygen isotope composition was analyzed. On the basis of the ages and the variations of the δ¹⁸O of the stalagmite, the climate change of the last 7.7 ka has been reconstructed in this area: 7.7–5.8 ka, summer monsoon maximum period; 5.8–3.8 ka summer monsoon weakening period; 3.8–0.15 ka, weakened summer monsoon and high amplitude climate fluctuations period. We put forward that the increasing trend of δ¹⁸O of the stalagmite reflected not only the weakening of the summer monsoon, which was caused by the decreasing of solar radiation in the Northern Hemisphere, but also the possibly decreasing contribution of the southwest monsoon to the rainfall of this area. These results are consistent with the output of the numerical climate-model modeling. The high amplitude fluctuation of the δ¹⁸O may imply the quick shift of the contributions of different moisture sources to the precipitation in this area.     

Summer monsoon and dust signals recorded in the Dasuopu firn core, central Himalayas

In September 1997, a 15-m firn core was recovered from an elevation of 7 000 m a. s.l. from the Dasuopu Glacier in the central Himalayas. The analysis of δ18O values and major ion (Ca2+ , Mg2+ , NH4+ , SO42- and NO3-) concentrations shows that average annual accumulation is 0.75 m (water equivalent) in the Dasuopu firn core. The seasonal variations of δ18O values and major ion concentrations in the core indicate that present summer monsoon and dust signals are recorded with high-resolution in the Dasuopu Glacier. δ18O in precipitation are controlled by amount effect in summer monsoon season, more negative δ18O is representative of summer monsoon signal in snow layers. Higher concentrations of Ca2+ , Mg2+ , SO42- and NO3-are dominated by spring dust storm imput derived from the arid and semi-arid desert regions in central Asia. Also EOF analysis verifies that high spring concentrations of major ions are consistent. Due to the possibly different sources, the secondary variations of NH4+ and NO3- are negatively relevant with that of Ca2+ and Mg2+ .     

Stalagmite-inferred Holocene precipitation in northern Guizhou Province,China, and asynchronous termination of the Climatic Optimum in the Asian monsoon territory

An absolute-dated, bi-decadal-resolution, stalagmite oxygen-isotopic time series from Shigao Cave reveals the evolution of summer monsoon precipitation over the past 9.9 ka BP in northern Guizhou Province, Southwest China. The  18O-inferred climate conditions are divisible into three distinct stages: (1) a maximum humid era from 9.9-6.6 ka BP; (2) a gradual declining precipitation interval between 6.6-1.6 ka BP; and (3) a relatively low precipitation time window after 1.6 ka BP. Consistency of contemporaneous stalagmite Holocene 18O records between Shigao and other caves in the Indian and East Asian monsoon realms support the effect of primary orbital solar forcings on monsoonal precipitation. However, statistical analysis shows a significant spatial asynchroneity of the Holocene Optimum termination in the Asian monsoon territory. The Holocene Optimum ended at 7.2-7.4 ka BP in Oman, located in the Indian monsoon region, and at 5.6-5.8 ka BP in Central China, in the East Asian monsoon zone. In Southwest China, the termination occurred between these periods, at 6.6-7.0 ka BP, and was influenced by both monsoon systems. We propose that this spatially asynchronous ending of Holocene Optimum in Asia may be attributed to sea surface temperature changes in the western tropical Pacific, which is a primary moisture source for the East Asian monsoon.     

Modern stalagmite oxygen isotopic composition and its implications of climatic change from a high-elevation cave in the eastern Qinghai-Tibet Plateau over the past 50 years

An oxygen isotope record of a stalagmite from Huanglong Cave in the eastern Qinghai-Tibet Plateau dated with 230Th and 210Pb methods provides variations of the Asian monsoon with an average resolu-tion of 1 year over the past 50 years. This study shows that the δ18O of dripwater in the cave represents the annual mean δ18O of local meteoric precipitation and the stalagmites were deposited in isotopic equilibrium. A comparison of the stalagmite δ18O record with instrumentally meteorological data indi-cates that shifts of the δ18O are largely controlled by the amount effect of meteoric precipitation con-veyed through the southwest monsoon(the Indian monsoon) and less affected by temperature. Therefore,the variations of δ18O record reflect the changes in monsoon precipitation on inter-annual time scales under the influence of the southwest monsoon. Like many other stalagmite δ18O records in the Asian monsoon regions,the δ18O record of the stalagmite from Huanglong Cave also reveals a gradually enriched trend during the past 50 years,i.e. relatively enriched in 18O. This trend may indicate the decline of the Asian monsoon intensity which is consistent with the decrease of monsoon indices. The weakening of the modern Asian monsoon well matched with the temperature changes in strato-sphere,which may illustrate that the weakening of the monsoon mainly results from the lowering of solar radiation.     

Isotopic composition of atmospheric water vapor before and after the monsoon’s end in the Nagqu River Basin

Atmospheric water vapor samples were collected in the Nagqu River Basin in the middle of Tibetan Plateau between August and October in 2004. Results show that there exist some fluctuations of the δ^18O of atmospheric water vapor, especially before and after the monsoon＇s end.Moreover, the variety trend of the δ^18O of atmospheric water vapor inverse correlates with that of dew point. Precipitation events make an important effect upon the variation of δ^18O of atmospheric water vapor. During the whole sampling period, the δ^18O values of atmospheric water vapor are low while precipitation events occurred. The moisture origins w also contribute to the variation of δ^18O of atmospheric water vapor. The oceanic moisture transported by the southwest monsoon results in lower δ^18O of atmospheric water vapor in the Nagqu River Basin. Compared with the influence of the oceanic moisture, the δ^18O values, however, appear high resuiting from the effect of the continental air mass in this region.     

Origin of summer monsoon rainfall identified by δ18O in precipitation

A negative correlation between δ^18O in monsoon precipitation and f, the ratio of precipitable water in monsoon region to that in water source area, is hypothesized. Using the Rayleigh model, a new method for identifying origin of summer monsoon rainfall is developed based on the hypothesis. In order to validate the method, the isotopic data at New Delhi, a typical station in the southwest monsoon region, and Hong Kong, a typical station in the southeast monsoon region, were collected and analyzed for case studies. The case studies indicate that the water source areas of the monsoon rairdall at the two stations identified by the method are accordant with the general atmosphere circulation patterns. The method developed in this paper is significantly important for tracing the origin of summer monsoon precipitation.     

Study of altitudinal lapse rates of δ18O in precipitation/river water with seasons on the southeast Tibetan Plateau

Seasonal δ^18O variation in water on the southeast Tibetan Plateau has been studied, showing the consistent variation pattern of δ^18O with altitude indicative of relevant atmospheric circulation processes. Study shows a similar variation pattern of fixed-site river water δ^18O with that of the precipitation δ^18O in southeast Tibet. δ^18O in regional rivers in southeast Tibet demonstrates a gradual depletion with increasing altitude, though the rates vary seasonally. The most depleted river ^18O occurs during the monsoon period, with the lowest δ^18O/altitude lapse rate. The river ^18O during the westerly period is also depleted, together with low δ^18O/altitude lapse rate. The pre-monsoon rivers witness the most enriched ^18O with least significant correlation coefficient with the linear regression, whilst the postmonsoon rivers witness the largest δ^18O/altitude lapse rate. Different coherence of seasonal δ^18O variation with the altitude effect is attributed to different moisture supplies. Though sampling numbers vary with seasons, the δ^18O-H linear correlation coefficients all reach the 0.05 confidence level, thus witnessing the variation features of δ^18O in seasonal river water due to the influence of atmospheric general circulation and land surface processes revealed from the altitudinal lapse rates.     

Transmission of oxygen isotope signals of precipitation-soil water-drip water and its implications in Liangfeng Cave of Guizhou,China

According to systemically monitoring results of oxygen （hydrogen） isotope compositions of precipitation, soil waters, soil CO2, cave drip waters and their corresponding speleothems in Liangfeng Cave （LFC） in Guizhou Province, Southwest China, it is found that local precipitation is the main source of soil waters and drip waters, and that the amplitudes of those δ18O values of three waters （precipitation, soil water and drip water） decrease in turn in the observed year, which are 0‰ to -10‰, -2‰ to -9‰ and -6‰ to -8‰, respectively. Moreover, the δ18O values for three waters show a roughly simultaneous variation, namely, that those values are lighter in the rainy seasons, weightier in the dry seasons, and that the average δ18O value of drip waters is about 0.3‰ weightier than that of precipitation, which is modified by surface evaporation processes. We also find that oxygen isotope equilibrium is reached or neared in the formation processes of speleothems in LFC system, and that it is feasible to reconstruct paleotemperature and paleoprecipitation by using δ18O values of speleothems. However, it should be noted that surface evaporation would affect the oxygen isotope values in the study area.     

The altitude effect of δ 18O in precipitation and river water in the Southern Himalayas

The lapse rate of water isotopes is used in the study of the hydrologic cycle as well as in the estimation of uplift of the Tibetan Plateau.The greater elevation contrast in the Southern Himalayas allows for a detailed discussion about this lapse rate.We analyze variations of 18δO in precipitation and river water between 1320 m and 6700 m elevations in the Southern Himalayas,and calculate the specific lapse rate of water 18δO.The results show that the multi-year average lapse rate in precipitation over this region is 0.15‰/100 m.The one-year average lapse rate is 0.17‰/100 m from three sites along the Southern Himalayas.The two results agree,but are much lower than the global average of 0.28‰/100 m.This work also shows that there is a difference in precipitation 18δO lapse rate between the monsoon and non-monsoon seasons.The calculated precipitation lapse rate is much lower than that in surface water.     

Simulations of seasonal variations of stable water isotopes in land surface process model CLM

In this study, we simulated and analyzed the monthly variations of stable water isotopes in different reservoirs at Manaus, Brazil, using the Community Land Model （CLM） that incorporates stable isotopic effects as a diagnostic tool for understanding stable water isotopic processes, filling the observational data gaps and predicting hydrometeorological processes. The simulation results show that the δ^18O values in precipitation, vapor and surface runoff have distinct seasonality with the marked negative correlations with corresponding water amount. Compared with the survey results by the International Atomic Energy Agency （IAEA） in co-operation with the World Meteorological Organization （WMO）, the simulations by CLM reveal the similar temporal distributions of the δ^18O in precipitation. Moreover, the simulated amount effect between monthly δ^18O and monthly precipitation amount, and MWL （meteoric water line） are all close to the measured values. However, the simulated seasonal difference in the δ^18O in precipitation is distinctly smaller than observed one, and the simulated temporal distribution of the δ^18O in precipitation displays the ideal bimodal seasonality rather than the observed single one. These mismatches are possibly related to the simulation capacity and the veracity in forcing data.     

Preliminary evidence indicating Dome A （Antarctica） satisfying preconditions for drilling the oldest ice core

Lowest temperature and snow accumulation rate are preconditions for retrieving the oldest ice core from the polar ice sheets. The 10-m depth firn temperature at Dome A, the summit of the Antarctic Ice Sheet, recorded by an automatic weather station （AWS） was -58.3℃ in 2005 and -58.2℃ in 2006, respectively. The 10-m firn temperature is an approximation of the annual mean air temperature （AMAT）, and this is the lowest AMAT that has been recorded on the surface of the Earth. The stable isotopic ratios （δ^18O and δD） of surface snow at Dome A are also lower than at other ice sheet domes along the East Antarctic Ice Divide such as Dome C, Dome F, Dome B and Vostok. These facts indicate that Dome A is the ＂pole of cold＂ on the Earth. The total amount of snow accumulation rate in 2005 and 2006 was only 0.16 cm, equaling 0.016 m water equivalent per year, the lowest precipitation ever recorded from Antarctica. Preliminary evidences indicate that Dome A is a candidate site for recovering the oldest ice core.     

Precipitation variability in central Himalayas and its relation to Northern Hemisphere temperature

Agriculture, industry and hydroelectric power in south Asia are heavily dependent on the performance of the summer (June to September) monsoon rainfall, which provides 75—90% of the annual rainwater over most parts of the area. A weak monsoon year generally corresponds to low crop yields. And strong monsoon usually produces abundant crops, although too much rainfall may produce devastating floods. However, modeling efforts to forecast the monsoon have met with only moderate success[1]. Prev…     

Isotopic evidence for the moisture origin and composition of surface runoff in the headwaters of the Heihe River basin

We investigated the moisture origin and contribution of different water sources to surface runoff entering the headwaters of the Heihe River basin on the basis of NECP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) re-analysis data and variations in the stable hydrogen and oxygen isotope ratios (?? D and ?? ¹⁸O) of precipitation, spring, river, and melt water. The similar seasonality in precipitation ?? ¹⁸O at different sites reveals the same moisture origin for water entering the headwaters of the Heihe River basin. The similarity in the seasonality of ?? ¹⁸O and d-excess for precipitation at Yeniugou and Urumchi, which showed more positive ?? ¹⁸O and lower d-excess values in summer and more negative ?? ¹⁸O and higher d-excess values in winter, indicates a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter. Higher d-excess values throughout the year for Yeniugou suggest that in arid inland areas of northwestern China, water is intensively recycled. Temporal changes in ?? ¹⁸O, ??D, and d-excess reveal distinct contributions of different bodies of water to surface runoff. For example, there were similar trends for ?? D, ?? ¹⁸O, and d-excess of precipitation and river water from June to September, similar ?? ¹⁸O trends for river and spring water from December to February, and similar trends for precipitation and runoff volumes. However, there were significant differences in ?? ¹⁸O between melt water and river water in September. Our results show that the recharge of surface runoff by precipitation occurred mainly from June to mid-September, whereas the supply of surface runoff in winter was from base flow (as spring water), mostly with a lower runoff amount.     

Long-term trend and abrupt events of the Holocene Asian monsoon inferred from a stalagmite δ18O record from Shennongjia in Central China

The middle-Holocene was a period of profound cul- ture transitions: the Neolithic culture around Central China[1], Mesopotamia[2] and India[3] all mysteriously collapsed around 4 ka . It is plausible that this civiliza-tion collapse can be contributed to …