Precipitation δ18O Research Articles

Continuous, near-real-time observations of water stable isotope ratios during rainfall and throughfall events

Abstract. The water isotopic composition of throughfall is affected by complex diffusive exchange with ambient water vapour, evaporative enrichment of heavy isotopes, and mixing processes in the tree canopy. All interception processes occur simultaneously in space and time, generating a complex pattern of throughfall depth and water isotopic composition. This pattern ultimately cascades through the entire hydrologic system and is therefore crucial for isotope studies in catchment hydrology, where recharge areas are often forested, while reference meteorological stations are generally in the open. For the quasi real-time observation of the water isotopic composition (δ18O and δ2H) of both gross precipitation and throughfall, we developed an approach combining a membrane contactor (Membrana) with a laser-based Cavity Ring-Down Spectrometer (CRDS, Picarro), obtaining isotope readings every 2 s. A setup with two CRDS instruments in parallel analysing gross precipitation and throughfall simultaneously was used for the continuous observation of the temporal effect of interception processes on the stable isotopes of water. All devices were kept small to minimize dead volume with time lags of only 4 min for water from the rainfall collectors to the isotope analysers to increase the temporal resolution of isotope observations. Complementarily, meteorological variables were recorded at high temporal resolution at the same location. The achieved evolution from discrete liquid or event-based bulk samples to continuous measurements allows for direct comparison of water stable isotope data with common meteorological measurements. Future improvements of the spatial representativeness will make our approach an even more powerful tool towards detailed insight into the dynamic processes contributing to interception during rainfall events.

Characteristics of Hydrogen and Oxygen Isotopes in Different Water Bodies in Hilly and Gully Regions of the Loess Plateau

Studying the isotope characteristics and water body transformation relationships among different water bodies in the hilly and gully region of the Loess Plateau can provide a theoretical foundation for evaluating regional climate, ecology, and water resources. In this study, daily and monthly averaged δD and δ18O in precipitation, river water, and shallow groundwater were measured in 2017 in the Jiuyuangou watershed, which has a good ecological condition. The compositional relationship between the stable hydrogen and oxygen isotopes in difference water bodies was explored, the influence factors and spatio-temporal variation of δ18O in precipitation and river water were analyzed, and the stable isotope conversion ratios between different water bodies in the study area were calculated using the two-terminal mixed model. The main conclusions of the study are as follows:the d-excess of river water showed an increasing trend with elevation during the observation period; the δ18O of river water was enriched with increasing distance from the river source and decreased with increasing altitude; temperature, wind speed, and relative humidity had significant effects on the hydrogen and oxygen isotopes of precipitation; precipitation and shallow groundwater replenish the river during the non-flood period, the proportions of which were 46% and 54%, respectively; and during the flood season, the shallow groundwater is replenished by river water and precipitation, the proportions were 60% and 40%, respectively. These results indicate that there is a good conversion relationship between "precipitation-river-shallow groundwater" in the study area. The implementation of soil and water conservation measures has had some influence on the conversion of different water bodies in small watersheds. The results provide a basis for the establishment of water cycle models for hilly and gully regions of the Loess Plateau.

Global analysis reveals climatic controls on the oxygen isotope composition of cave drip water

The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ18O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ18O is most similar to the amount-weighted precipitation δ18O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ18O records the recharge-weighted δ18O. This implies that the δ18O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.

Variation in depth of water uptake for Pinus sylvestris var. mongolica along a precipitation gradient in sandy regions

Mongolian pine (Pinus sylvestris var. mongolica) has been widely used in vegetation restoration and for windbreaks in sandy regions of northern China where water is the principal factor limiting tree survival and growth. An understanding of water use in Mongolian pine plantations is critical for effective vegetation restoration. To determine water sources in a Mongolian pine plantation, we investigated the stable isotopic ratios of δ18O and δD in precipitation, groundwater, in soil water in different soil layers, and in tree xylem at different tree ages along a precipitation gradient and in different micro-landforms. The results indicated that the main water sources for Mongolian pine were precipitation-derived shallow soil water in semi-humid regions and that the contribution proportion decreased with an increase in tree age. With reduced precipitation in semi-arid and arid regions, contribution of deep soil water to Mongolian pine water use gradually increased, and differences in soil water contribution among trees due to tree age also significantly increased. Water use patterns in trees planted in different micro-landforms (e.g. sand dune crest or inter-dune lowland) became more distinct as precipitation gradually decreased across humid to semiarid and arid regions. Moreover, Mongolian pine, including 15-, 20-, 25-, and 45-year-old trees, similar height and DBH without groundwater in semi-arid climate (in Ejin Horo banner), while trees planted in extremely arid climate (Linze) relied on irrigation for survival despite abundant and available groundwater at that study site. These results indicated that groundwater may be beyond reach to be a main water source for Mongolian pine trees. Soil water conditions determined the survival and sustainable growth of planted trees, especially at depths less than 1 m where soil layers were recharged mainly by precipitation. Although Mongolian pine exhibits strong ecological adaptability, a trait that makes it ideally-suited for controlling desertification in sandy regions of northern China, water stress, crown dieback, or even tree mortality are likely to occur under conditions of extremely low precipitation.

Stable isotope analysis of white-tailed deer teeth as a paleoenvironmental proxy at the Maya site of La Joyanca, northwestern Petén, Guatemala

ABSTRACTCarbon and oxygen isotopes ratios from herbivore teeth have previously been used as paleo-environmental proxies in temperate zones. However, their utility in tropical zones remains uncertain. In this study, sequential sub-samples from white-tailed deer (Odocoileus virginianus) teeth (second and third molars) from the Maya archaeological site of La Joyanca, located in northwestern Petén, Guatemala, show that δ18O of enamel carbonate corresponds broadly to modern observed precipitation δ18O over the 10-month period of tooth formation, capturing rainfall seasonality. The analyses also detect significant diachronic differences in the δ18O between the periods 1100–1000 BP (850–950 A.D.) and 1000–900 BP (950–1050 A.D.) at La Joyanca. The δ13C in both periods are indicative of a C3-plant based diet, which suggests cultivation of maize did not differentially affect deer diet during this period.

Stable isotopes characteristics of precipitation over Shaanxi-Gansu-Ningxia and its water vapor sources.

Based on hydrogen and oxygen stable isotopes in precipitation and meteorological data over Shaanxi-Gansu-Ningxia provided by the Global Network of Isotopes in Precipitation (GNIP) and in previous literature, the spatial and temporal variations of oxygen stable isotopes in precipitation and their driving factors were analyzed, the local meteoric water line (LMWL) functions were established. The results showed that the slope and intercept of the LMWL changed in the order of Gansu<Shaanxi<Ningxia, with all of them being lower than slope and intercept of the Global Meteo-ric Water Line. The precipitation process of three provinces had been evaporated and fractionated to different degrees. The evaporation was enhanced along Ningxia, Shaanxi and Gansu. The δ18O value in precipitation in the Shaanxi-Gansu-Ningxia region was enriched in summer and autumn but poor in winter and spring. For the spatial distribution, the weighted δ18O value decreased from the northwest to the southeast. δ18O in precipitation showed significant temperature effect, but was no precipi-tation effect, reflecting the characteristics of continental climate in the middle and high latitudes. The quantitative relation of elevation effect was -0.12‰·(100 m)-1, and latitude effect was more significant by -0.27‰ per °. The water vapor source was tracked by HYSPLIT model. The cluster trajectory of air masses showed that water vapor from the Bay of Bengal, southeast monsoon and westerly zone were the main sources in summer half year, while it mainly transported by the westerly belt in winter half year.

Influence of Summer Sublimation on δD, δ18O, and δ17O in Precipitation, East Antarctica, and Implications for Climate Reconstruction From Ice Cores

AbstractIn central Antarctica, where accumulation rates are very low, summer sublimation of surface snow is a key element of the surface mass balance, but its fingerprint in isotopic composition of water (δD, δ18O, and δ17O) remains unclear. In this study, we examined the influence of summer sublimation on δD, δ18O, and δ17O in precipitation using data sets of isotopic composition of precipitation at various sites on the inland East Antarctica. We found unexpectedly low δ18O values in the summer precipitation, decoupled from surface air temperatures. This feature can be explained by the combined effects of weak or nonexistent temperature inversion and moisture recycling associated with sublimation‐condensation processes in summer. Isotopic fractionation during the moisture‐recycling process also explains the observed high values of d‐excess and 17O‐excess in summer precipitation. Our results suggest that the local cycle of sublimation‐condensation in summer is an important process for the isotopic composition of surface snow, water vapor, and consequently precipitation on inland East Antarctica.

Abrupt changes in Indian summer monsoon strength during the last deglaciation and early Holocene based on stable isotope evidence from Lake Chenghai, southwest China

Identifying variability and the mechanisms driving variability in the Indian summer monsoon (ISM) since the last deglaciation is critical for understanding past hydroclimatic change. In this study, we present an accelerator mass spectrometry radiocarbon dated record of δ18O variations in authigenic carbonates derived from Lake Chenghai in southwest China, a region that receives moisture mainly from the Indian Ocean. The δ18O values vary from −11.9 to +0.1‰, providing a detailed record of variations in ISM precipitation δ18O values, and lake hydrological balance. The record shows that the ISM generally strengthened in the post-glacial between 15.6 and 8.8 cal ka BP, but that three centennial to millennial-scale drought events were superimposed on the long-term trend. Drought events, as indicated by substantial positive shifts in δ18O value, occurred from 15.6 ± 0.2 to 14.4 ± 0.2, 12.5 ± 0.2 to 11.7 ± 0.2 and 10.1 ± 0.1 to 10.0 ± 0.1 cal ka BP, corresponding to the Heinrich 1, Younger Dryas, and Bond 7 cold events in the North Atlantic region, respectively. The timings of these droughts are suggested to be related to meltwater discharge into the North Atlantic. The weakened Atlantic Meridional Overturning circulation, which leads in turn to the southward migration of the intertropical convergence zone and cooling in the tropical Indian Ocean.

Expanding the Isotopic Toolbox to Track Monarch Butterfly (Danaus plexippus) Origins and Migration: On the Utility of Stable Oxygen Isotope (δ18O) Measurements

The measurement of naturally occurring stable hydrogen (δ2H) and carbon (δ13C) isotopes in wings of the eastern North American monarch butterflies (Danaus plexippus) have proven useful to infer natal origins of individuals overwintering in Mexico. This approach has provided a breakthrough for monarch conservation because it is the only viable means of inferring origins at continental scales. Recently, routine simultaneous analyses of tissue δ2H and δ18O of organic materials has emerged leading to questions of whether the dual measurement of these isotopes could be used to more accurately infer spatial origins even though the two isotopes are expected to be coupled due to the meteoric relationship. Such refinement would potentially increase the accuracy of isotopic assignment of wintering monarchs to natal origin. We measured a sample of 150 known natal-origin monarchs from throughout their eastern range simultaneously for both δ2H and δ18O wing values. Wing δ2H and δ18O values were correlated (r2=0.42). We found that wing δ2H values were more closely correlated with amount-weighted growing season average precipitation δ2H values predicted for natal sites (r2=0.61) compared to the relationship between wing δ18O values and amount-weighted growing season average precipitation δ18O values (r2=0.30). This suggests that monarch wing δ2H values will be generally more useful in natal assignments than δ18O values. Spatial information related to the use of deuterium excess in environmental waters was similarly found to be not useful when applied to monarch wings likely due to the considerable variance in wing δ18O values. Nonetheless, we recommend further testing of monarch wing δ2H and δ18O values from known natal sites with an emphasis on field data across a strong gradient in precipitation deuterium excess.

Stable carbon and oxygen isotopic composition of modern land snails along a precipitation gradient in the mid-latitude East Asian monsoon region of China

The stable oxygen and carbon isotope composition (δ18Oshell and δ13Cshell) of land snail shells preserved in Chinese eolian loess deposits have the potential to reconstruct past changes in the East Asian Summer Monsoon (EASM) since the late Oligocene. To assess the potential of δ18Oshell and δ13Cshell for the semi-quantitative reconstruction of past variations of the EASM, we studied modern snails and the corresponding vegetation composition of three distinct ecosystems (woodland, grassland and mountains), spanning a large rainfall gradient (178–1403 mm) in the mid-latitude East Asian monsoon region. The results show that different snail species in the three ecosystems have significantly different values of δ18Oshell and δ13Cshell. The snails in woodland from the eastern plain area have significantly lower δ18Oshell and δ13Cshell values than those of snails in grassland from the Chinese Loess Plateau, and the snails from the Qinba Mountains region have values intermediate between the two. Systematic variations of the δ13C of snail shells, body tissues and local plants are evident along the rainfall gradient, indicating that δ13Cshell is mainly influenced by the δ13C of ingested plants, which is controlled by the EASM. We confirm that δ13Cshell is a reliable precipitation proxy and we obtained a new empirical equation (δ13Cshell = −0.0030(±0.0003) MAP − 8.80(±0.32), r = −0.76, n = 74, p < 0.001) for the relationship between δ13Cshell and mean annual precipitation, which is based on the results of the present study, together with those of Bao et al. (2018). We infer that δ18Oshell is influenced by precipitation δ18O, and that here is a significant relationship between δ18Oshell and mean annual precipitation (δ18Oshell = −0.0028(±0.0008) MAP − 1.32(±0.58), r = −0.65, p < 0.05, n = 21), possibly because of the rainfall amount effect in the study region. In addition, temperature and evaporation associated with relative humidity play non-negligible roles in determining δ18Oshell, and their effects should be considered before using the δ18O of fossil snail shells to reconstruct precipitation δ18O and/or rainfall amount.

Characteristics of Stable Isotopes in Precipitation and Moisture Sources in the Headwaters of the Yangtze River

Based on the stable isotopes of 73 precipitation samples continuously collected from May to October 2014 and related meteorological statistics in the Dongkemaldi Basin, the characteristics of δD, δ18O, and d-excess of precipitation, as well as the correlations between δ18O and the rainfall amount and air temperature were analyzed. The moisture sources were tracked by the HYSPLIT model to further estimate the contribution of different water vapor sources to the rainfall amount. The results showed that the range of δ18O and δD values varied from -26.5‰ to 1.9‰ and -195.2‰ to 34.0‰, respectively; meanwhile, the δ18O and δD values in precipitation fluctuated greatly with time in response to water vapor transport from different moisture sources of the Qinghai-Tibet Plateau. The slope and intercept of the Local Meteoric Water Line (LMWL) were both higher than those of the Global Meteoric Water Line (GMWL) and close to the LMWL in the northern area of the Qinghai-Tibet Plateau. The relationship between δ18O and δD in different precipitation types showed significant differences, which were mainly related to the source of water vapor and meteorological conditions during the process of precipitation formation. Because of the influence of local evaporation and the transport process of water vapor, the d-excess values of atmospheric precipitation were relatively large; the δ18O in precipitation had a significant amount effect, but had no temperature effect, thus indicating that the rainfall amount was more effective in controlling the stable isotope content of atmospheric precipitation than temperature. The modeled trajectory of vapor sources showed that water vapor of precipitation was mainly derived from the marine vapor carried by the southwest monsoon, local moisture, and the westerly water vapor, and their contributions to the rainfall amount were 43%, 36%, and 21%, respectively. The results of this study can contribute to further understanding of the atmospheric circulation characteristics and water cycle process of the Dongkemadi basin in the headwaters of the Yangtze River.

Event‐Based Precipitation Isotopes in the Laurentian Great Lakes Region Reveal Spatiotemporal Patterns in Moisture Recycling

AbstractLake effect snowstorms influence climate, ecology, and agriculture in the Laurentian Great Lakes region and can be costly to surrounding communities in the United States and Canada. Stable isotopes of lake effect precipitation events throughout the year display a distinct signature that can be used to better understand how these storms may respond to a changing climate. Here we present event‐based δ18O and δ2H of precipitation from a site in Skaneateles, NY, downwind of Lakes Ontario and Erie, between April 2015 and February 2018. We find a seasonal isotopic cycle with a well‐defined signature of high deuterium excess (d‐excess) during National Weather Service‐defined lake effect snowstorms. Application of a previously developed moisture recycling model to this data set shows that up to 25% more moisture recycling takes place when the lake water and air temperature difference is large, air temperature is below freezing, and wind direction permits storms to move over Lake Ontario or Lake Erie. Moisture recycling occurs less frequently during spring, summer, and early fall due to meteorological and lake parameters that are less conducive to moisture recycling. Comparison of annual mean precipitation d‐excess at sites both upwind and downwind of the Laurentian Great Lakes provides evidence that this high d‐excess signature is characteristic of mean annual precipitation isotopic composition at downwind sites and therefore may be used to quantify changes in moisture recycling that occur on event to annual time scales in response to past and future climate changes.

Influence of the North Atlantic Oscillation on δD and δ18O in meteoric water in the Armenian Highland

The stable hydrogen (δD) and oxygen (δ18O) isotope composition of modern meteoric water are influenced by changes in global temperature, Rayleigh distillation processes, and moisture transport history. However, on a global scale the relative influence of these different meteorological variables is spatially heterogeneous. We collected daily precipitation samples from meteorological stations in the Armenian Highlands for a period of one year in order to determine the constraints on δD and δ18O and relationship to broader climatic conditions. These data are coupled with HYSPLIT back-trajectory analysis to determine how these compositions relate to moisture source and transport path. Multivariate analysis shows the strongest relationship between δD and δ18O and climate variables is temperature at the sampling site. Along with temperature, the intensity of the North Atlantic Oscillation, a pattern of atmospheric circulation which determines the strength and location of Northern Hemisphere midlatitude westerlies, is significant (p < 0.05) for both δ18O and δD values of precipitation. This correlation between NAO and isotopes in precipitation is also observed in four nearby GNIP stations from 2008 to 2015. Negative NAO phases are associated with weakened westerlies, which allows precipitation enriched in heavy isotopes from the Mediterranean and southerly sources to penetrate into the Armenian Highlands, whereas positive NAO phases are associated with stronger westerlies bringing more negative precipitation from the Black Sea. This relationship is most significant (p < 0.001) during the winter and spring months (December to May). These data demonstrate that paleo-archives of δD and δ18O in the Armenian Highlands could serve as a tool to measure changes in the NAO in the past, particularly when coupled with independent measurements of temperature.

Hydroclimatic seasonality recorded by tree ring δ18O signature across a Himalayan altitudinal transect

Water stable isotope ratios of tropical precipitation predominantly reflect moisture source and precipitation intensity. Trees can incorporate the isotopic signals into annual tree-ring cellulose records, permitting reconstruction of the temporal changes of hydroclimate over decades to millennia. This is especially valuable in the Himalayas where the understanding of monsoon dynamics is limited by the lack of a dense and representative observational network. We have analyzed tree ring δ18O records from two distinct physiographic sites along the upper Kali Gandaki valley in the central Nepal Himalayas, representing the wet High-Himalayas and the Trans-Himalayan dryland to the north. Empirical correlations and regression analyses were compared to an in-situ calibrated oxygen isotope fractionation model, exploring the relationships between tree ring δ18O and seasonal-mean variability of hydroclimatic forcing at the different locations. For this purpose, gridded precipitation data from the Asian rain gauge dataset APHRODITE, as well as high resolution onsite observations (relative humidity, air temperature, δ18O of precipitation and radial tree growth) were used. We found that two distinct sets of meteorological values, reflecting pre-monsoon and monsoon conditions, are needed to reproduce the measured tree ring δ18O values from the High-Himalayan site, but that a single set of monsoonal values performs best for the Trans-Himalayan site. We conclude that Trans-Himalayan trees capture long-term changes in strength of the Indian summer monsoon. In contrast, High-Himalayan tree ring δ18O records a more complex hydro-climatic signal reflecting both pre-monsoon and monsoon seasons with very contrasting isotopic signatures of precipitation. This difference in the two hydroclimatic proxy records offers an opportunity to reconstruct first-order hydroclimate conditions, such as local precipitation rates, and to gain new insights into monsoon timing and seasonal water source determination across the Himalayan orographic region.

Triple oxygen isotope signatures of evaporation in lake waters and carbonates: A case study from the western United States

Evaporation can increase the δ18O values of lake waters and carbonates by several per mil. If not accounted for in geological studies, this can lead to substantial misinterpretation of δ18O values in terms of paleoclimate and paleoelevation. Evaporation also leads to a lowering in residual waters of Δ17O, a measure of the departure of δ′17O from a characteristic relationship with δ′18O. We present new triple oxygen isotope data from waters and carbonates from lakes and their source rivers in the western United States (Bear Lake, Great Salt Lake, Lake Tahoe, Mono Lake, and Pyramid Lake). Consistent with predictions from steady-state isotopic mass balance models, the data illustrate marked lowering of Δ17O in closed basin lakes and freshwater lakes relative to their source rivers. The evaporation slope in triple oxygen isotope space (λlake) is similar for these lakes, averaging 0.5229 and ranging between 0.5219 and 0.5239. Moreover, models and data both show that the evaporation slope correlates with Δ17O, meaning that the slope can be estimated on the basis of the measured Δ17O value of the carbonate. We show how triple oxygen isotopes in lake waters and carbonates and ‘clumped isotopes’ (Δ47) in carbonates can be combined to reconstruct the δ18O values of primary (unevaporated) catchment precipitation (δ18Orucp). We use our lacustrine carbonate data as a test case for this approach, and find that δ18Orucp values closely approximate independently-measured δ18O values of catchment precipitation. However, the δ18Orucp values are consistently higher than δ18O of catchment precipitation by ∼2‰, which may reflect present incomplete understanding of a number of triple oxygen isotope parameters used in the calculation, such as the fractionation exponent for carbonate-water equilibrium, the evaporation slope λlake, and the Δ17O values of unevaporated meteoric waters. In conclusion, triple oxygen isotope analysis of lake waters and lacustrine carbonates is a promising new method for studying evaporation in fossil lake systems, but will benefit from additional research into triple oxygen isotope systematics relevant to meteoric waters, lake systems, and carbonate–water fractionation.

Tracing the Relationship between Precipitation and River Water in the Northern Carpathians Base on the Evaluation of Water Isotope Data

The aim of this study is to investigate the stable isotope composition of precipitation and river water from the northeastern part of Romania. For this study, we collected monthly samples (for variable periods of time) of precipitation from six stations, and river water from three stations, between March 2012 and December 2017. The precipitation in the area is sourced mainly from the Atlantic Ocean, and secondarily from the Black Sea, local recycling being important especially in summer. We found that the seasonal δ18O in precipitation is in agreement with the seasonal temperature variability, as shown by the significant correlation coefficient between the two variables (r = 0.77), which indicates that the temperature has an important role in the δ18O variability in precipitation water in this region. The local meteoric water line in the northeastern part of Romania is defined by the equation δ2H = 7.80 × δ18O + 7.47, (r2 = 0.99, n = 121). The results presented in this study emphasize that the δ18O (and δ2H) and d-excess variability are strongly influenced by temperature, precipitation and the prevailing large-scale atmospheric circulation.

Subseasonal Variations of Stable Isotopes in Tropical Andean Precipitation

Abstract The tropical Andes of southern Peru and northern Bolivia have several major mountain summits suitable for ice core paleoclimatic investigations. However, incomplete understanding of the controls on the isotopic (δD, δ18O) composition of precipitation and a paucity of field observations in this region continue to limit ice-core-based paleoclimate reconstructions. This study examines four years of daily observations of δD and δ18O in precipitation from a citizen scientist network on the northeastern margin of the Altiplano, to identify controls on the subseasonal spatiotemporal variability in δ18O during the wet season (November–April). These data provide new insights into modern δ18O variability at high spatial and temporal scales. We identify a regionally coherent subseasonal signal in precipitation δ18O featuring alternating periods of high and low δ18O of 9–27-day duration. This signal reflects variability in precipitation delivery driven by synoptic conditions and closely relates to variations in the strength of the South American low-level jet and moisture availability over the study area. The annual layer of snowpack on the Quelccaya Ice Cap observed in the subsequent dry season retains this subseasonal signal, allowing the development of a snow-pit age model based on precipitation δ18O measurements, and demonstrating how synoptic variability is transmitted from the atmosphere to mountaintop snowpacks along the Altiplano’s eastern margin. This result improves our understanding of the hydrometeorological processes governing δ18O and δD in tropical Andean precipitation and has implications for improving paleoclimate reconstructions from tropical Andean ice cores and other paleoclimate records.

Variation characteristics of stable isotopes in precipitation and the environmental factors that influence them in the Shiyang River Basin of China

Stable isotopes in precipitation are used to understand the hydrologic cycle as well as changes in climate. Based on 274 precipitation samples collected from 6 stations in the Shiyang River Basin from July 2013 to June 2014, the variation characteristics of stable isotopes and their response to the environment contribute to the understanding of the hydrologic cycle in an arid region of Northwest China. Stable isotopes in the Shiyang River Basin vary significantly with the seasons. Compared to the winter months, the slope and intercept of the Local Meteoric Water Line (LMWL) are lower and the values of δ18O and δD are higher in the summer. The variations of δ18O and δD in precipitation are significantly affected by temperature, elevation, and sub-cloud evaporation, but are not affected by precipitation on an annual scale. The effects of temperature are more pronounced in the summer than in the winter. Isotope variations are weakly affected by precipitation amounts during the summer, especially in July, from both greater precipitation and continuous precipitation. The concentrations of δ18O and δD decrease with height at rates of − 0.3‰/100 m and − 2‰/100 m, respectively. In the summer months, the values of δ18O and δD at the surface are greater than those at cloud base, indicating that the variations of stable isotopes are influenced by sub-cloud evaporation. In general, higher values of Δ18δ and Δ2δ are related to higher temperatures, less precipitation, and lower relative humidity, with the reverse also being true. The variations of stable isotopes and the environmental factors that influence them differ with meteorological conditions and geography.

Dynamic characteristics and influencing factors of precipitation δ18O, China

It is the basis of isotope technology application to clarify which factors are related to precipitation isotopes in different regions. China has a vast territory, and there is still no comprehensive and systematic understanding of the spatial distribution pattern and influencing factors of precipitation stable isotopes in China. Based on the known Bowen and Wilkinson model, the latitude and altitude of 117 sites in China were used to simulate δ18O value of each site and to generate the high precision spatial distribution map of the precipitation oxygen stable isotope over China. According to the spatial distribution of precipitation δ18O, the basic effects of isotope variation in different climatic regions were analyzed: latitude effect, elevation effect, temperature effect, precipitation effect, and water vapor source and transportation process. In the eastern monsoon region, the latitude effect of isotope variation is shown, but the difference between the southern region and the northern region is large. The southern region is affected by the water vapor of the subtropical Pacific Ocean and precipitation amount effect to form a high value area of δ18O, and the northern region is more susceptible to the temperature effect. In the same latitude area, due to the influence of local recycled water vapor and local evaporation, a high-value area of δ18O is formed in the inland basin of the northwestern arid region. In the Qinghai–Tibet Plateau, the isotope values affected by the high terrain are generally low. The change of δ18O value reflects the extent to which the south and north areas of the Qinghai–Tibet Plateau are affected by the southwest monsoon and local water vapor.

Stable isotope variations in precipitation in the northwesternmost Tibetan Plateau related to various meteorological controlling factors

Precipitation plays an important role in global water cycle, and the stable hydrogen and oxygen isotopes of precipitation provide useful information in hydrological processes. To evaluate the controlling factors on stable isotopes of precipitation in the northwesternmost Tibetan Plateau (NWTP), the spatial-temporal variation of δ18O and δ2H in precipitation was investigated based on 120 event-based precipitation samples collected from 3 stations (Jiangka, Shaman and Xihexiu) during 2011–2014 and 45 monthly average precipitation data of Hetian station (GNIP) and 23 individual precipitation events of Shiquanhe station during 2001. The values of δ18O and δ2H in precipitation are more positive in the summer (June to July) and negative in the winter (November to March). The seasonal d-excess pattern observed in the NWTP is remarkably different from that in adjacent regions. The local meteoric water line (LMWL) was calculated as δ2H = 8.22 δ18O + 12.41 (R2 = 0.98, n = 194) based on the event-based samples. Temperature and altitude effects show significant influence on the precipitation δ18O in the study area. Sub-cloud evaporation and recycling moisture also are important controlling factors during the precipitation process. The sub-cloud evaporation decreased d-excess between 1.7% and 45.8%, and the d-excess decreased linearly with increasing sub-cloud evaporation at about 1.15‰ per 1% change of sub-cloud evaporation. The recycled moisture has an important influence on the precipitation on the Xihexiu, Shaman and Jiangka stations. The result of this study is helpful to the understanding of regional water cycling and optimal water resource management during water shortages.