River Water δ18O Research Articles

Spatial variation of water stable isotopes of multiple rivers in southeastern Qaidam Basin, northeast Qinghai-Tibetan Plateau: Insights into hydrologic cycle

River contains plenty of information on hydrological cycle. The water stable isotope approach is a powerful tool to unravel the hydrological processes such as river water sources, evaporation and river water-groundwater interaction. However, the spatial variation and influencing factors of stable isotope compositions in multiple rivers within a large basin and their indication to hydrological cycle have been less documented. This work reports on the spatial variation of stable water isotopic compositions of river water of four rivers and the underlying hydrological processes in the southeastern Qaidam Basin, northeast Qinghai-Tibetan Plateau, China. The results show that the δD and δ18O values of individual river vary along the river flow directions (generally from south to north), and that of multiple rivers also exhibit an obvious spatial variation, which is mainly influenced by evaporation, groundwater discharge, moisture source and transport pattern and local precipitation. As for an individual river, the δD and δ18O of river water are gradually enriched by evaporation along the river flow direction, which are rapidly depleted due to groundwater discharge into river in the groundwater overflow zone. For multiple rivers, the stable isotopic compositions of river water are gradually depleted from the west to the east of the study area (i.e. approximately perpendicular to rivers’ flow directions), which are mainly controlled by moisture source and transport pattern. This study helps to enhance our understanding of the hydrological cycle in a large basin through combined analysis of water stable isotopic data from individual river and multiple rivers. The data evaluation methods and results of this study could be useful in other areas to understand hydrological processes and moisture transport pattern.

δ2H and δ18O of river water from a high-altitude humid plain of the southern Alps: Implications for the interpretation of the isotopic compositions of bioapatite from humans living close to mountain areas

Located in the French Southern Alps, the Cerveyrette area is a watershed of about 100 km2 including a torrent and a high-altitude swampy plain dominated by a nival regime. The apparent cyclicity of very small magnitude in the δ18O (−14.4 ± 0.6 ‰) of the river waters is interpreted to partly reflect seasonal variations in the isotopic compositions of precipitations, thus having consequences on the interpretation of human and animal remains of archaeological interest. As the Cerveyrette is the potential main source of drinking water for local humans and animals, considerations about the meaning of calculated theoretical δ18O of bioapatite (bone, tooth enamel) highlight that inferred Mean Air Annual Temperatures (MAAT) are highly underestimated, with a calculated MAAT of 1.5 °C instead of the local recorded MAAT of 7 °C. In terms of human or animal mobility, such isotopic data may drive researchers to look for a distant and high-latitude geographic origin instead of a rather close and high-altitude mountainous areas. For example, the area where the famous Ötzi lived, discovered close to the Austrian-Italian border, could have been of lower altitude than previously thought. Consequently, we propose that the interpretation of bioapatite δ18O data must be undertaken with caution in mountainous areas, especially with the consideration of the regional orography and its related hydrographic networks.

Hydrogeochemical characteristics and recharge sources identification based on isotopic tracing of alpine rivers in the Tibetan Plateau

Alpine rivers originating from the Tibetan Plateau (TP) contain large amounts of water resources with high environmental sensitivity and eco-fragility. To clarify the variability and controlling factors of hydrochemistry on the headwater of the Yarlung Tsangpo River (YTR), the large river basin with the highest altitude in the world, water samples from the Chaiqu watershed were collected in 2018, and major ions, δ2H and δ18O of river water were analyzed. The values of δ2H (mean: −141.4‰) and δ18O (mean: −18.6‰) were lower than those in most Tibetan rivers, which followed the relationship: δ2H = 4.79*δ18O-52.2. Most river deuterium excess (d-excess) values were lower than 10‰ and positively correlated with altitude controlled by regional evaporation. The SO42− in the upstream, the HCO3− in the downstream, and the Ca2+ and Mg2+ were the controlling ions (accounting for >50% of the total anions/cations) in the Chaiqu watershed. Stoichiometry and principal component analysis (PCA) results revealed that sulfuric acid stimulated the weathering of carbonates and silicates to produce riverine solutes. This study promotes understanding water source dynamics to inform water quality and environmental management in alpine regions.

Surface water isoscapes (δ18O and δ2H) reveal dual effects of damming and drought on the Yangtze River water cycles

The water regimes and biogeochemical cycles are remarkably affected by river damming and drought around the world. However, the dual effects on river water cycles have not yet attracted sufficient attention along the Yangtze River because of the relative consistency of annual water regimes. This study first explored the altered water cycle associated with damming and drought using surface water isoscapes (δ18O and δ2H) of river and lake water along the Yangtze River during the flooding seasons of 2016 and 2018, combined with other historical isotopic data from 2003 to 2013. The results show that river water isotopes are markedly varied on both spatial and temporal scales. The relatively more enriched river water representing δ18O values were observed from 2003 to 2007 compared with those in 2018, which were possibly caused by increased evaporative fractionation under abnormal drought conditions in the Yangtze River. Spatially, river water δ18O values exhibited a positive trend from upstream to the lower reaches because of the contrasting recharged sources, complex mixing, and evaporation from upstream to the lower reaches of the Yangtze River. This trend was related to variations in surface water temperature and water evaporation during the flooding seasons evidenced by a significant correlation between river isotopes and river water temperature. Moreover, the large variability in deuterium-excess (d-excess) and δ18O in the downstream could be inherited from the upstream because of short residence time and incomplete mixing in the Three Gorges Reservoir (TGR) during the initial stage of the Three Gorges Dam (TGD) operation (2003–2007). In contrast, the relatively lower variability of d-excess and δ18O was found in the downstream area due to the complete mixing in the TGR, causing the homogenization of reservoir water during the normal operation stage after 2010 (e.g., 2013 and 2018). Drought stimulates evaporative fractionation on river water and result in more negative d-excess values especially in 2006. The dual effects of river damming and drought remarkably changed the longitudinal hydrological connectivity and water cycle dynamics by prolonging the residence time and enhancing the water mixing, which should be thoroughly considered for water resource management, especially for regions with heavily-regulated rivers.

Distribution of hydrogen and oxygen stable isotopes and pollution indicators in water during a monsoon transitional period in Min River Basin

Based on 197 monthly river water and groundwater samples and 30 event-scale precipitation samples, our study reports the distribution of hydrogen and oxygen isotopes and pollution indicators in Min River Basin. The variation of δ18O and d-excess indicate that the water source in the upper main course water is more variable and that in the middle-lower part is relatively stable. Comparison between plots of δ2H versus δ18O in the river water and precipitation reflect the dominant water source is different between river water in the upper and middle-lower parts. The electrical conductivity (EC) shows a similar spatial variation trend for main course water collected in four campaigns. The pollutant concentration change at the confluences of main tributaries shows that the inflow of Heishui River and Dadu River leads to decreased NO3− and Cl−, while that of Xi River, Pu River and Fuhe River leads to a leap in NO3− and Cl−. A significant positive correlation is observed between EC and δ18O, indicating the consistent control of water sources on isotope distribution and water quality. The relationship between elevation and δ18O in the main course river water suggests that the factors affecting isotope distribution vary spatially. “Altitude effect” can only be observed in October and November for the upper steepest plateau zone due to the spatial variation in the precipitation stored during the wet season. The “inverse altitude effect” is observed for the upper part during the wet season and for the middle-lower part during the whole study period, which can be explained by the contribution of tributaries with different discharge regimes. Our findings show that water source with different discharge regimes can serve as the leading factor controlling the stream component in multi-tributary river basins with large spatial span and may mask the influence of spatial distribution of precipitation.

Characterizing spatial and temporal variation in 18O and 2H content of New Zealand river water for better understanding of hydrologic processes

AbstractTime series of hydrogen and oxygen stable isotope ratios (δ2H and δ18O) in rivers can be used to quantify groundwater contributions to streamflow, and timescales of catchment storage. However, these isotope hydrology techniques rely on distinct spatial or temporal patterns of δ2H and δ18O within the hydrologic cycle. In New Zealand, lack of understanding of spatial and temporal patterns of δ2H and δ18O of river water hinders development of regional and national‐scale hydrological models. We measured δ2H and δ18O monthly, together with river flow rates at 58 locations across New Zealand over a two‐year period. Results show: (a) general patterns of decreasing δ2H and δ18O with increasing latitude were altered by New Zealand's major mountain ranges; δ2H and δ18O were distinctly lower in rivers fed from higher elevation catchments, and in eastern rain‐shadow areas of both islands; (b) river water δ2H and δ18O values were partly controlled by local catchment characteristics (catchment slope, PET, catchment elevation, and upstream lake area) that influence evaporation processes; (c) regional differences in evaporation caused the slope of the river water line (i.e., the relationship between δ2H and δ18O in river water) for the (warmer) North Island to be lower than that of the (cooler, mountain‐dominated) South Island; (d) δ2H seasonal offsets (i.e., the difference between seasonal peak and mean values) for individual sites ranged from 0.50‰ to 5.07‰. Peak values of δ18O and δ2H were in late summer, but values peaked 1 month later at the South Island sites, likely due to greater snow‐melt contributions to streamflow. Strong spatial differences in river water δ2H and δ18O caused by orographic rainfall effects and evaporation may inform studies of water mixing across landscapes. Generally distinct seasonal isotope cycles, despite the large catchment sizes of rivers studied, are encouraging for transit time analysis applications.

Spatial variations of hydrochemistry and stable isotopes in mountainous river water from the Central Asian headwaters of the Tajikistan Pamirs

Water resources in Central Asia from the mountainous headwater catchments is changing due to the shrinkage of glaciers in the Tian Shan and Pamir mountain systems. In order to predict future changes in water quality, it is crucial to understand what factors are governing the spatial variations of water chemistry and hydrological processes in mountainous headwater catchments. In this study, water chemistry including major ions and stable isotopes in the headwaters of major Tajikistan rivers was studied. Results showed that Tajikistan river water had an alkaline pH value (mean: 8.2) and total dissolved solids (mean: 368.5mg/L) were higher than the global average value. Ca2+, Na+, HCO3−, and SO42− in the rivers were the most abundant cations and anions, controlled by the rock weathering process and evaporation-crystallization processes. The hydrochemical facies of river water was dominated by Ca-HCO3 (71.7%) and exhibited spatial heterogeneity, which was related to the lithologic compositions and water source across Tajikistan. A significant negative correlation of river water δ18O with elevation was observed with a vertical lapse rate of 0.17‰/100 m. The more negative δ18O values in rivers from eastern Tajikistan were scattered in the lower left corner of the δ18O-δ2H plot, implying that the rivers were primarily supplied by snow/glacier meltwater because of the substantial number of glaciers and high elevation mountain in eastern regions. The drinking and irrigation suitability from ionic compositions revealed that the water quality of Tajikistan rivers was naturally good, though some sites posed a safety concern. These findings can provide new insights into sustainable management of water quality in the climatically and lithologically distinct segments of headwater regions in the Tajikistan Pamirs.

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.

Spatial distribution and controlling factors of surface water stable isotope values (δ18O and δ2H) across Kazakhstan, Central Asia

Climate change is expected to alter hydrological and biogeochemical processes in Central Asia (CA), and surface water stable isotope values (δ18O and δ2H) can be used to examine these changes. Spatially extensive stable isotope data, however, are sparse, which constrains the understanding of hydrological processes in transboundary rivers across Kazakhstan. Therefore, we conducted a survey of surface water stable isotopes across the region. River and lake water isotope values exhibit spatial variability that was closely associated with isotope values of precipitation, physiographic factors, landscape characteristics, and local hydrological processes, e.g., evaporation and mixing of waters from different sources. River water was characterized by lower δ18O and δ2H values and higher d-excess relative to lake water, suggesting evaporative enrichment of lake water. Analysis of δ18O versus δ2H for rivers and lakes yielded distinct regressions, (river [RWL], δ2H = 6.08δ18O − 16.7, r2 = 0.837, p < 0.001) and (lake [LWL], δ2H = 6.23δ18O − 22.1, r2 = 0.924, p < 0.001). The slope and intercept of the RWL and LWL were slightly lower than the local meteoric water line [LMWL] (δ2H = 6.96 δ18O − 1.0, r2 = 0.942, p < 0.001). River water δ18O showed a significant negative correlation with elevation and longitude, but not with latitude. The spatial distributions of δ18O and d-excess values showed a remarkable gradient from west to east across Kazakhstan that was associated with moisture moving from the Mediterranean region to Kazakhstan. We also found generally higher δ18O values and lower d-excess values in low-elevation areas because of high evaporation rate in Kazakhstan. These baseline data will be useful for documenting the effects of climate change on the hydrological cycle in Central Asia.

Spatial and temporal variability of stable isotopes (δ18O and δ2H) in surface waters of arid, mountainous Central Asia

AbstractCharacterization of spatial and temporal variability of stable isotopes (δ18O and δ2H) of surface waters is essential to interpret hydrological processes and establish modern isotope–elevation gradients across mountainous terrains. Here, we present stable isotope data for river waters across Kyrgyzstan. River water isotopes exhibit substantial spatial heterogeneity among different watersheds in Kyrgyzstan. Higher river water isotope values were found mainly in the Issyk‐Kul Lake watershed, whereas waters in the Son‐Kul Lake watershed display lower values. Results show a close δ18O–δ2H relation between river water and the local meteoric water line, implying that river water experiences little evaporative enrichment. River water from the high‐elevation regions (e.g., Naryn and Son‐Kul Lake watershed) had the most negative isotope values, implying that river water is dominated by snowmelt. Higher deuterium excess (average d = 13.9‰) in river water probably represents the isotopic signature of combined contributions from direct precipitation and glacier melt in stream discharge across Kyrgyzstan. A significant relationship between river water δ18O and elevation was observed with a vertical lapse rate of 0.13‰/100 m. These findings provide crucial information about hydrological processes across Kyrgyzstan and contribute to a better understanding of the paleoclimate/elevation reconstruction of this region.

Stable and clumped isotope sclerochronologies of mussels from the Brazos River, Texas (USA): Environmental and ecologic proxy

Oxygen isotope sclerochronology with mollusk shells can provide vital environmental and ecologic information, but is complicated in regulated, subtropical rivers by irregular river water δ18O and consequently irregular shell δ18O patterns. We performed multi-proxy analyses (δ18O, δ13C, clumped isotopes) from serially-sampled freshwater mussels in two specimens each of Amblema plicata and Cyrtonaias tampicoensis from the Brazos River, Texas to age specimens and test whether they accurately record environmental conditions such as river discharge and water source. Oxygen isotopic measurements are similar to predicted aragonite δ18O values based on temperature and water δ18O, but the record is complex and irregular. To better resolve the chronologies, we performed clumped isotope analyses on select shell intervals. Clumped isotope temperatures (T(Δ47)) ranged from 19° to 36 °C. Summer T(Δ47) values were 3–5 °C higher than measured temperatures, suggesting an offset in paleothermometer calibration. Chronologies based on Δ47 and δ18O reveal specimen ages of 3–4 years and winter growth cessation (or dramatic slowing) in both C. tampicoensis and A. plicata, highlighting thermal limitations on growth and potential biases in environmental reconstructions. Compared with modern shells, historical shells collected before intensive damming showed lower δ18O values yet similar clumped temperatures, suggesting enhanced evaporation in modern river water due to impoundment and/or the 2011–12 drought.Carbon isotope trends were similar between conspecific shells, but differed between species, with substantially higher average values for A. plicata shells (−9‰) than C. tampicoensis shells (−12‰). The correlation between shell δ13C values and the release of 13C-enriched waters from upstream reservoirs demonstrates that δ13C can be used as a proxy for dam release in regulated rivers.

Response of surface water quantity and quality to agricultural water use intensity in upstream Hutuo River Basin, China

A field hydrology survey was conducted to determine spatial variations in regional water yield and use intensity and clarify the main causes of the variation in upstream Hutuo River Basin (UHTB). The study showed that: 1) the spatial distribution of flow, salt and stable isotopes reflected variations in regional water yield and use intensity. Total Dissolved Solids (TDS) in river water were lower in Qingshui tributary and the headwater region of Hutuo River (HTR). The distributions of δD and δ18O of river water were nearly parallel to the Local Meteoric Water Line (LMWL), except for the middle reach. Intensive agricultural activities were responsible for the deviation of spatially stable isotopes from LMWL, the highest TDS and 0.85 evaporation fraction in the middle reach. 2) The spatial flow indicated that almost all the river water was used in agriculture (especially in the middle reach of UHTB) before inflow from Qingshui tributary. Based on the balance in monitored flow rate and isotopes, about 67.9% of the water flow (11.66 m3/s) was from upstream Shanxi province and discharged into the downstream of Hebei Province; 91.9% of which was from Qingshui tributary. 3) Irrigation function of upstream reservoirs was not fully reflected due to less storage of water along with low recharge and discharge events. Given the declining water yield and high water requirement, there was the need to overhaul agricultural water use strategies to ensure cross-border water availability. The research could lay the basis for designing cross-border agricultural water use strategies in UHTB and other basins with similar hydro-climatic conditions.

Spatiotemporal distribution of river water stable isotope compositions and variability of lapse rate in the central Rocky Mountains: Controlling factors and implications for paleoelevation reconstruction

Stable isotope-based paleoaltimetry is the most widely used approach for paleoelevation reconstruction. Interpretations of stable isotope data in continental interiors, however, are undermined by surface water isotope compositions that are influenced by multiple factors. Here we present a stable isotope dataset of modern river water samples collected over two summers and one spring from the central Rocky Mountains (Rockies) and the adjacent Great Plains. By examining the spatial and temporal variations of river water δ18O, δD and d-excess values, and their relationships with climatic and geographic parameters, as well as through back trajectory analysis of moisture sources, we elucidate the influences of elevation and climatic parameters on the spatiotemporal variation of river water isotopic values. In the Bighorn River drainage, a typical intermontane drainage in the central Rockies, the isotopic difference between highland and lowland rivers is small, which we attribute to highland precipitation that dominates lowland river discharge. In the North Platte River drainage across the central Rockies and Great Plains, the river water δ18O values show poor correlation with elevation west of 105°W (central Rockies), but increase as elevation decrease east of 105°W (in the western Great Plains). This eastward increase across the western Great Plains leads to an average oxygen isotope lapse rate of −2.3‰/km, which we interpret as being caused primarily by condensation temperature-controlled isotopic fractionation at various elevations, and secondarily by evaporation in the upper reaches of streams that contribute to the North Platte River plus direct contribution of moisture from the Gulf of Mexico in the Great Plains. In this continental interior setting, multiple moisture sources, including recycled continental moisture, contribute to surface water, and evaporation influences river water isotope values to various degrees depending on the relative humidity within an individual river catchment. These results suggest that paleoclimate and atmospheric circulation pattern must be carefully evaluated when applying stable isotope-based paleoaltimetry in continental interiors. Our findings have implications for paleoelevation reconstruction in the study area, including that 1) within the central Rockies, the isotopic difference of river water and unevaporated basinal precipitation can be used to infer paleorelief of the Laramide ranges with respect to the basin floors; 2) along a regional transect crossing the central Rockies and Great Plains, the modern isotope lapse rate of the North Platte River drainage can be used to constrain the paleorelief between the two regions in semi-arid climate.

Differential isotopic characteristics of eco‐hydrologic processes in a subtropical watershed, China

AbstractIn order to better understand isotopic characteristics of eco‐hydrologic processes in subtropical zones, this study selected a typical subtropical watershed in China to investigate and determine the complex differential mechanisms inherent to this water body type. Results showed that stable isotopes in precipitation within a Local Meteoric Water Line in the Xiangxi River basin (where δD = (8.23 ± 0.27)δ18O + (17.97 ± 1.65); R2 = 0.94) weighted δD and δ18O average precipitation values were −22.11‰ and −5.23‰, respectively. Furthermore, enrichment trends in river water δ18O (−4.21‰ to −6.91‰) and δD (−25.11‰ to −38.13‰) and soil solution δ18O (−6.30‰ to −11.21‰) and δD (−11.21‰ to −34.57‰) were greater compared with precipitation δ18O (−0.23‰ to −12.06‰) and δD (0.54‰ to −98.52‰) and throughfall δ18O (−1.11‰ to −4.72‰) and δD (−2.11‰ to −24.95‰). This study found obvious seasonality and higher deuterium excess (d‐excess) values in precipitation during the dry season and lower values during the rainy season. Moreover, the orchard was more enriched and the coniferous and broad‐leaved mixed forest was more depleted in δD and δ18O compared with other land cover types. Precipitation was an important climate variable in subtropical watersheds under the influence of monsoon systems. According to results from the isotopic mixture model, precipitation input fractions were greater than 88% in the soil solution and greater than 21% in river water.

Spatial variation of stable isotopic composition in surface waters of the Huai River basin, China and the regional hydrological implication

AbstractOxygen (δ18O) and hydrogen (δD) stable isotopes in the surface waters of the Huai River basin were analyzed in this study. Results indicated the northern waters had higher δ18O and δD than the southern waters, the water δ18O and δD increased along the water flow directions. These variations mostly resulted from the spatial differences of precipitation and evaporation. Comparing with published different continents' river water δ18O data, this study suggests that evaporation effect is a more plausible interpretation than altitude effect as the cause of δ18O increasing from upriver to downriver waters. This region's local surface water line (LSWL, δD = 5.36δ18O − 18.39; r2 = 0.84) represents one of the first presented LSWLs in eastern China. The correlation between d-excess and δ18O demonstrates this region is dominated by the Pacific oceanic moisture masses in summer. Comparing the various LSWLs from eastern China and eastern United States river waters, this study proposes a hypothesis that the water LSWLs slopes of lower latitude regions may be less than those of higher latitude regions within similar topographic areas. This hypothesis may be tested in other geographically comparable coupled areas in the world if corresponding large-scale data can be found.

Water isotopes and hydrograph separation in different glacial catchments in the southeast margin of the Tibetan Plateau

AbstractSnow and glaciers are known to be important sources for freshwater; nevertheless, our understanding of the hydrological functioning of glacial catchments remains limited when compared with lower altitude catchments. In this study, a temperate glacial region located in the southeast margin of the Tibetan Plateau is selected to analyse the characteristics of δ18O and δD in different water sources and the contribution of glacier–snow meltwater to streamflow. The results indicate that the δ18O of river water ranges from −16.2‰ to −10.2‰ with a mean of −14.1‰ and that the δD values range from −117.0‰ to −68.0‰ with a mean of −103.1‰. These values are more negative than those of glacier–snow meltwater but less negative than those of precipitation. The d‐excess values are found to decrease from meltwater to river to lake/reservoir water as a result of evaporation. On the basis of hydrograph separation, glacier–snow meltwater accounts for 51.5% of river water in the Baishui catchment in the melting season. In the Yanggong catchment, snow meltwater contributes 47.9% to river water in the premonsoon period, and glacier meltwater contributes only 6.8% in the monsoon period. The uncertainty in hydrograph separation is sensitive to the variation of tracer concentrations of streamflow components. The input of meltwater to a water system varies with local climate and glacier changes. The results confirm that hydrograph separation using water isotopes is valuable for evaluating the recharge sources of rivers, especially in ungauged glacial regions. This study provides insights into the hydrological processes of glacial catchments on the Tibetan Plateau, which is important for water resource management.

Laminated tufa sediments formed from overflow karst springs: Controls on their deposition and carbon–oxygen isotope records

AbstractTufa sediments are freshwater carbonates that precipitate in karst regions after degassing of carbon dioxide from groundwater in contact with the atmosphere. When laminated, these carbonates can provide high‐resolution records for the study of climate, hydrological and environmental conditions at the time of their precipitation. The formation of these carbonates directly depends on the hydrological regime, and in karst regions discontinuous discharges are often recorded. This study investigates the record of recent laminated tufa sediments precipitated downstream overflow springs in Trabaque Canyon (central Spain). The hydrological dynamics of the karst system were monitored for over three years and a stable isotope record was obtained from laminated tufa carbonates precipitated from an overflow spring. Additionally, a hydrological model of overflow springs was generated and a tufa δ18O record under constrained parameters was simulated. Temperature is the dominant control of the variation in tufa δ13C and δ18O values within each lamina, although when comparing different laminae, δ13CDIC and δ18O of river water are also major controls. The positive correlation between tufa δ13C values and water temperature is caused by the fractionation occurred by carbon dioxide degassing due to the thermal dependence of carbon dioxide solubility. Additionally, the system recorded a temperature‐independent degassing process caused by the large gradient between groundwater and atmospheric carbon dioxide that is limited to the proximity of the spring. This study cautions on the risk of assuming continuous deposition when studying laminated tufa sediments and highlights the potential of their stable isotope records to provide hydrological information of their aquifers during the past.

A study of longitudinal and altitudinal variations in surface water stable isotopes in West Pamir, Tajikistan

In an effort to establish a clear relation between stable isotopes and altitude in the Pamir region, as well as to improve the understanding of stable isotope spatial variations found along the routes taken by the westerlies, surface river water samples were collected along a roughly west–east profile in Tajikistan. Here we present the spatial changes in modern surface water δ18O and deuterium excess (d-excess), and their links with snow melt, glacier melt and seasonal precipitation patterns in Tajikistan. The results show a close relation between river water δ18O and δD above the GMWL (Global Meteoric Water Line), implying that surface evaporation exerts a weaker influence. Spatially, river water δ18O is gradually depleted from west to east due mainly to the effect of altitude, yielding a river water δ18O vertical lapse rate of 0.09‰/100m. River water d-excess shows a decreasing trend from west to east in Tajikistan. This spatial d-excess pattern in surface water is explained by the different moistures influenced by the Mediterranean and other in-land seas with higher d-excess in west, and no-Mediterranean moisture in east. Another possible reason is the differences in precipitation seasonality between the west and east of Tajikistan.

Characteristics of water isotopes and ice-snowmelt quantification in the Tizinafu River, north Kunlun Mountains, Central Asia

The isotopic composition of river water, precipitation, and ice-snowmelt water have been measured in samples simultaneously collected at 29 sampling sites located at the junction of the principal river course and its tributaries. Distribution of stable isotopes, and temporal and spatial variations of δD and δ18O of river water and the sources of the Tizinafu River were investigated. The δ18O values of river water samples were near the GMWL and LMWL, which indicated similarity in constitution of water sources. We found significant seasonal effect in the water isotope, with maximum values in summer, and minimum values in winter. The d-excess changes in the opposite sense. Response of water isotope and d-excess towards temperature and elevation were assessed by correlation analysis. A negative relationship between elevation and δ18O was found, whereas little relationship was found for the elevation and d-excess. Furthermore, we quantitatively determined the ratio of ice-melt water using isotope hydrograph separation. Results showed the contribution of ice-snowmelt water varied from 25.96% to 68.87% for spatial characteristics, and from 28.31% to 65.43% for seasonal characteristics. The mean of the ice-snowmelt percentage is 43%, which meant that ice-snowmelt water was the main supplying water source for the Tizinafu River. The roles of ice-snowmelt water should be noted in water resource management in glacial rivers.

Indian monsoon influences altitude effect of δ18O in precipitation/river water on the Tibetan Plateau

The δ18O 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 δ18O 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 δ18O in precipitation on the Plateau is influenced by different moisture sources, with significant Indian monsoon influence on δ18O composition in plateau precipitation and river water. The δ18O 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 δ18O 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 δ18O 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 δ18O in precipitation and river water on the Tibetan Plateau results from the combined effect of monsoon moisture supply and westerly moisture supply, the δ18O composition and its altitude effect on the Plateau during monsoon seasons should be considered in the reconstruction of paleoelevation of the Tibetan Plateau.