Isotopic composition of precipitation in a topographically steep,seasonally snow‐dominated watershed and implications of variations from the global meteoric water line

The local meteoric water line (LMWL), the functional relationship between locally measured values of δ¹⁸O and δ²H in precipitation, represents the isotopic composition of water entering hydrologic systems. The degree to which the LMWL departs from the global meteoric water line (GMWL), moreover, can reveal important information about meteoric sources of water (e.g. oceanic or terrestrial) and atmospheric conditions during transport. Here we characterize the isotopic composition of precipitation within an experimental watershed in the Western US that is subject to large topographic and seasonal gradients in precipitation. Interpreting the hydrometeorologic and spatial controls on precipitation, we constructed a seasonally weighted LMWL for southwestern Idaho that is expressed by the equation δ²H = 7.40 × δ¹⁸O ? 2.17. A seasonally weighted LMWL that is based on weighting isotopic concentrations by climatic precipitation volumes is novel, and we argue better represents the significant seasonality of precipitation in the region. The developed LMWL is considerably influenced by the semiarid climate experienced in southwest Idaho, yielding a slope and y‐intercept lower than the GMWL (δ²H = 8 × δ¹⁸O + 10). Moderate to strong correlations exist between the isotopic composition of precipitation from individual events and surface meteorologic variables, specifically surface air temperature, relative humidity, and precipitation amount. A strong negative correlation exists between the annual average isotopic composition of precipitation and elevation at individual collection sites, with a lapse rate of ?0.22‰/100 m. Copyright © 2016 John Wiley & Sons, Ltd.     

From precipitation to runoff: stable isotopic fractionation effect of glacier melting on a catchment scale

Stable isotope variability and fractionation associated with transformation of precipitation/accumulation to firn to glacial river water is critical in a variety of climatic, hydrological and paleoenvironmental studies. This paper documents the modification of stable isotopes in water from precipitation to glacier runoff in an alpine catchment located in the central Tibetan Plateau. Isotopic changes are observed by sampling firnpack profiles, glacier surface snow/ice, meltwater on the glacier surface and catchment river water at different times during a melt season. Results show the isotopic fractionation effects associated with glacier melt processes. The slope of the δD‐δ¹⁸O regression line and the deuterium excess values decreased from the initial precipitation to the melt‐impacted firnpack (slope from 9.3 to 8.5 and average d‐excess from 13.4‰ to 7.4‰). The slope of the δD‐δ¹⁸O line further decreased to 7.6 for the glacier runoff water. The glacier surface snow/ice from different locations, which produces the main runoff, had the same δD‐δ¹⁸O line slope but lower deuterium excess (by 3.9‰) compared to values observed in the firnpack profile during the melt season. The δD‐δ¹⁸O regression line for the river water exhibited a lower slope compared to the surface snow/ice samples, although they were closely located on the δD‐δ¹⁸O plot. Isotope values for the river and glacier surface meltwater showed little scatter around the δD‐δ¹⁸O regression line, although the samples were from different glaciers and were collected on different days. Results indicate a high consistency of isotopic fractionation in the δD‐δ¹⁸O relationships, as well as a general consistency and temporal covariation of meltwater isotope values at the catchment scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Seasonal variations of deuterium and oxygen‐18 isotopes and their response to moisture source for precipitation events in the subtropical monsoon region

Deuterium and oxygen‐18 are common environmental tracers in water used to investigate hydrological processes such as evaporation and groundwater recharge, and to trace moisture source. In this study, we collected event precipitation from 01 January 2010 to 28 February 2011 at a site in Changsha, Yangtze River Basin to estimate the influence of moisture source and atmospheric conditions on stable isotope compositions. The local meteoric water line, established as δD = (8.45 ± 0.13) δ¹⁸O + (17.7 ± 0.9) (r² = 0.97, n = 189), had a higher slope and intercept than global meteoric water line. Temperature–δ¹⁸O exhibited complex correlations, with positive correlations during Nov.–Apr. superior to during Jun.–Sep., which was attributed to distinctive moisture sources, but vague the overall period; amount effect examined throughout the year. Linear regressions between δ¹⁸O and δD value in different precipitation event size classes revealed progressively decreasing slope and intercept values with decreasing precipitation amount and increasing vapour pressure deficit, indicating that small rainfall events (0–5 mm) were subject to secondary evaporation effects during rainwater descent. In contrast, snowfall and heavy precipitation events exhibited high slope and intercepts for the regression equation between δ¹⁸O and δD. High concentrations of heavy isotopes were associated with precipitation events sourced from remote westerly air masses, degenerated tropical marine air masses from the Bay of Bengal (BoB), and inland moisture in the pre‐monsoon period, as determined from backward trajectories assessed in the HYSPLIT model. Meanwhile, low concentrations of heavy isotopes were found to correspond with remote maritime moisture from BoB, the South China Sea, and the west Pacific at three different air pressures in summer monsoon and post‐monsoon using HYSPLIT and records of typhoon paths. These findings suggest that stable isotope compositions in precipitation events are closely associated with the meteorological conditions and respond sensitively to moisture source in subtropical monsoon climates. Copyright © 2013 John Wiley & Sons, Ltd.     

Groundwater Origin in Qanats,Chemo-Isotopic,and Hydrogeological Evidence

A systematic study of the chemo-isotopic characteristics and origin of the groundwater was carried out at six major qanats in the hyper-arid Gonabad area, eastern Iran. These qanats as a sustainable groundwater extraction technology have a long history, supporting human life for more than a thousand years in this region. The Gonabad qanats are characterized by outlet electrical conductivity (EC) values of 750 to 3900 µS/cm and HCO₃-Na-Mg and Cl-Na water types. The Gonabad meteoric water line (G_nMWL) was drawn at the local scale as δ²H = 6.32×δ¹⁸O + 8.35 (with R² = 0.90). It has a lower slope and intercept than the global meteoric water line due to different water vapor sources and isotope kinetic fractionation effects during precipitation in this arid region. The altitude effects on isotopic content of precipitation data were derived as δ¹⁸O = (−0.0031 × H_(m.a.s.l))−1.3). The δ²H and δ¹⁸O isotopes signatures demonstrate a meteoric origin of the groundwater of these qanats. The shift of the qanat's water samples from the local meteoric water line (LMWL) in a dry period with higher temperatures is most probably due to evaporation during the infiltration process and water movement in qanat gallery. Based on the isotopic results and mass balance calculations, the qanats are locally recharged from an area between 2000 to 2400 m.a.s.l of nearby carbonate formations and coarse alluvial sediments. The dissolution of evaporate interlayers in Neogene deposits deteriorates the groundwater quality, especially in Baidokht qanat.     

Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain,Nevada, USA,revealed by the isotope composition of fluid-inclusion water

Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ¹⁸O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ¹⁸O values were calculated from the δ¹⁸O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry.The δD values measured in all samples range between ? 110 and ? 90‰, similar to Holocene meteoric water. Coupled δ¹⁸O–δD values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive δ¹⁸O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 °C), which is only possible in the deep-seated hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin.     

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

Stable water isotopes (δ¹⁸O and δ²H) are an important source signature for understanding the hydrological cycle and altered climate regimes. However, the mechanisms underlying atmospheric water vapour isotopes in the northeast Qinghai‐Tibetan Plateau of central Asia remain poorly understood. This study initially investigated water vapour isotopic composition and its controls during the premonsoon and monsoon seasons. Isotopic compositions of water vapour and precipitation exhibited high variability across seasons, with the most negative average δ¹⁸O values of precipitation and the most positive δ¹⁸O values of water vapour found during the premonsoon periods. Temperature effect was significant during the premonsoon period but not the monsoon period. Both a higher slope and intercept of the local meteoric water line were found during the monsoon period as compared with in the premonsoon period, suggesting that raindrops have been experienced a greater kinetic fractionation process such as reevaporation below the cloud during the premonsoon periods. The δ²H and δ¹⁸O signatures in atmospheric water vapour tended to be depleted with the occurrence of precipitation events especially during the monsoon period and probably as a result of rainout processes. The monthly average contribution of evaporation from the lake to local precipitation was 35.2%. High d‐excess values of water vapour were influenced by the high proportion of local moisture mixing, as indicated by the gradually increasing relative humidity along westerly and Asian monsoon trajectories. The daily observation (observed ε) showed deviations from the equilibrium fractionation factors (calculated ε), implying that raindrops experienced substantial evaporative enrichment during their descent. The average fraction of raindrops reevaporation was estimated to be 16.4± 12.9%. These findings provide useful insights for understanding the interaction between water vapour and precipitation, moisture sources, and help in reconstructing the paleoclimate in the alpine regions.     

Isotopic composition (δ18O and δD) of precipitation and groundwater in a semi‐arid,mountainous area (Guadiana Menor basin,Southeast Spain)

We characterize the precipitation and groundwater in a mountainous (peaks slightly above 3000 m a.s.l.), semi‐arid river basin in SE Spain in terms of the isotopes ¹⁸O and ²H. This basin, with an extension of about 7000 km², is an ideal site for such a study because fronts from the Atlantic and the Mediterranean converge here. Much of the land is farmed and irrigated both by groundwater and runoff water collected in reservoirs. A total of approximately 100 water samples from precipitation and 300 from groundwater have been analysed. To sample precipitation we set up a network of 39 stations at different altitudes (800–1700 m a.s.l.), with which we were able to collect the rain and snowfall from 29 separate events between July 2005 and April 2007 and take monthly samples during the periods of maximum recharge of the aquifers. To characterize the groundwater we set up a control network of 43 points (23 springs and 20 wells) to sample every 3 months the main aquifers and both the thermal and non‐thermal groundwater. We also sampled two shallow‐water sites (a reservoir and a river). The isotope composition of the precipitation forms a local meteoric water line (LMWL) characterized by the equation δD = 7·72δ¹⁸O + 9·90, with mean values for δ¹⁸O and δD of − 10·28‰ and − 69·33‰, respectively, and 12·9‰ for the d‐excess value. To correlate the isotope composition of the rainfall water with groundwater we calculated the weighted local meteoric water line (WLMWL), characterized by the equation δD = 7·40δ¹⁸O + 7·24, which takes into account the quantity of water precipitated during each event. These values of (dδD/dδ¹⁸O)< 8 and d‐excess (δD–8δ¹⁸O)< 10 in each curve bear witness to the ‘amount effect’, an effect which is more manifest between May and September, when the ground temperature is higher. Other effects noted in the basin were those of altitude and the continental influence. The isotopic compositions of the groundwater are represented by the equation δD = 4·79δ¹⁸O − 18·64. The groundwater is richer in heavy isotopes than the rainfall, with mean values of − 8·48‰ for δ¹⁸O and − 59·27‰ for δD. The isotope enrichment processes detected include a higher rate of evaporation from detrital aquifers than from carbonate ones, the effects of recharging aquifers from irrigation return flow and/or from reservoirs' leakage and enrichment in δ¹⁸O from thermal water. Copyright © 2010 John Wiley & Sons, Ltd.     

Seasonal connections between meteoric water and streamflow generation along a mountain headwater stream

In snowmelt-driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ¹⁸O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high-relief mountainous catchments. We characterized δ¹⁸O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ¹⁸O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow-derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ¹⁸O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ¹⁸O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late-winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid-summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions.     

Hydroclimatic significance of stable isotopes in precipitation from glaciers of Garhwal Himalaya,Upper Ganga Basin (UGB), India

Stable isotopic composition of precipitation as preserved in continental proxy climate archives (e.g., ice cores, lacustrine sediments, tree rings, groundwater, and organic matter) can sensitively record fluctuations in local meteorological variables. These are important natural climatic tracers to understand the atmospheric circulation patterns and hydrological cycle and to reconstruct past climate from archives. Precipitation was collected at Dokriani Glacier to understand the response of glaciers to climate change in the Garhwal Central Himalaya, Upper Ganga Basin. The local meteoric water line deviates from the global meteoric water line and is useful for the identification of moisture source in the region. The data suggest different clusters of isotopic signals, that is, summer (June–September) and winter (November–April); the mean values of δ¹⁸O, δD, and d ‰ during summer are ?13.03‰, ?84.49‰, and 19.78 ‰, respectively, whereas during winter, the mean values of δ¹⁸O, δD, and d ‰ are ?7.59‰, ?36.28‰, and 24.46 ‰, respectively. Backward wind trajectory analysis ascertains that the major source of precipitation during summer is from the Indian Summer Monsoon and during winter from the westerlies. Regression analysis has been carried out in order to establish interrelationship between the precipitation isotopic signatures and meteorological variables such as air temperature, relative humidity, and precipitation. Temperature and precipitation have good correlation with the isotopic signatures of precipitation with R² values >.5, suggesting that both temperature and amount effects prevail in the study region. Multiple regression analysis found strong relationships for both the seasons. The relationship of deuterium excess with δ¹⁸O, relative humidity, and precipitation are significant for the winter season. No significant relationships of deuterium excess were found with other meteorological variables such as temperature and radiation. The correlation and regression analysis performed are significant and valuable for interpretation of processes in the hydrological cycle as well as for interpretation of palaeoclimate records from the region.     

Oxygen and hydrogen isotope studies of plutonic granitic rocks

The primary δD values of the biotites and hornblendes in granitic batholiths are remarkably constant at about ?50 to ?85, identical to the values in regional metamorphic rocks, marine sediments and greenstones, and most weathering products in temperate climates. Therefore the primary water in these igneous rocks is probably not “juvenile”, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducted lithosphere. Most granitic rocks have δ¹⁸O = +7.0 to +10.0, probably indicating significant involvement of high-¹⁸O metasedimentary or altered volcanic rocks in the melting process; such an origin is demanded for many other granodiorites and tonalites that have δ¹⁸O = +10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ¹⁸O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-¹⁸O magmas probably formed by melting or assimilation of hydrothermally altered roof rocks. However, the water/rock ratios were typically low enough that over wide areas the only evidence for meteoric water exchange in the batholiths is given by low D/H ratios (δD as low as ?180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves north through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks have δD values lower than ?120. The lowering of δD values commonly correlates with re-setting of K-Ar ages, and in the Idaho batholith two broad zones (10,000 km²) can be defined where δD biotite <?100 and K-Ar “ages” have all been re-set to values less than 60 m.y., suggesting that the Ar loss was caused by the meteoric-hydrothermal circulation systems. In certain Precambrian batholiths, a much different type of very low-temperature, regional alteration by surface-derived waters took place over an extended period long after emplacement, producing “brick-red” feldspars and markedly discordant Rb-Sr isochron “ages”.     

Analyzing Effects of Shrub Canopy on Throughfall and Phreatic Water Using Water Isotopes,Western China

Alpine shrub Quercus aquifolioides was selected to study the effects of shrub canopy on throughfall and phreatic water by analyzing the isotopic time series of precipitation, canopy throughfall and phreatic water and examining correlations among these series in Wolong Nature Reserve, Western China. Based on analysis of precipitation data in 2003, the local meteoric water line during the rainy season was δD = 8.28 × δ¹⁸O + 8.93, and the primary precipitation moisture in this region originated from the Pacific Ocean in the summer. Stable isotope analysis showed that the main supply of throughfall and phreatic water was from precipitation, and the shrub canopy has an important effect on the processes of rainwater transmuted into throughfall and phreatic water. Moreover, the differences of δD and δ¹⁸O values between rainwater and throughfall were relevant to rainfall. Due to interception of the shrub canopy, there had a response hysteresis of phreatic water to the various rainfall events, which was mostly 2 days, except that this hysteresis was ≤1 day when rainfall was >15 mm/day.     

Using stable isotopes to quantify water sources for trees and shrubs in a riparian cottonwood ecosystem in flood and drought years

Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ¹⁸O and δ²H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ¹³C values of leaf tissue.     

The effect of refreezing on the isotopic composition of melting snowpack

The isotopic composition of solid and liquid portions of natural melting snowpack is investigated in detail by the separating of liquid water from snow grains at different depths of the snowpack. The slope of the δD–δ¹⁸O line for the liquid phase is found to be lower than for the solid phase. This is proved to be due to the isotopic fractionation occurring in the melt–freeze mass exchange within the snowpack. Melting of the snowpack has no clear impact on the δD–δ¹⁸O line for the solid phase, but the slope of the δD–δ¹⁸O line for the liquid shows an overall slight decrease in the melting period. When the snowpack is refrozen, the refreezing process would inevitably cause the slope of the solid phase to decrease because of the discrepancy between the slopes of the two phases. Thus the slope of the solid would become lower and lower as the diurnal melt–freeze episodes cycle throughout the melting season. This effect is then demonstrated by looking into the isotopic composition changes of glacier firn. The extent of the effect depends on the snowpack properties and environmental conditions. The slope changes also result in a decreasing trend in deuterium excess. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatial and temporal patterns of stable water isotopes along the Yangtze River during two drought years

Changes in the level of the Yangtze River caused by anthropogenic water regulation have major effects on the hydrological processes and water cycle in surrounding lakes and rivers. In this study, we obtained isotopic evidence of changes in the water cycle of Yangtze River during the two drought years of 2006 and 2013. Isotopic evidence demonstrated that the δ¹⁸O and δD levels in Yangtze River exhibited high spatial heterogeneity from the upper to lower reaches, which were controlled by atmospheric precipitation, tributary/lake water mixing, damming regulation, and water temperature. Both the slope and intercept of Yangtze River evaporative line (δD = 7.88 δ¹⁸O + 7.96) were slightly higher than those of local meteoric water line of Yangtze River catchment (δD = 7.41 δ¹⁸O + 6.01). Most of the river isotopic values were located below the local meteoric water line, thereby implying that the Yangtze River water experienced a certain degree of evaporative enrichment on isotopic compositions of river water. The high fluctuations in the isotopic composition (e.g., deuterium excess [d‐excess]) in the middle to lower reaches during the initial stage of operation for the Three Gorges Dams (2003–2006) were due to heterogeneous isotopic signatures from the upstream water. In contrast to the normal stage (after 2010) characterized by the maximum water level and largest water storage, a relatively small variability in the deuterium excess was found along the middle to lower reaches because of the homogenization of reservoir water with a longer residence time and complete mixing. The effects of water from lakes and tributaries on the isotopic compositions in mainstream water were highlighted because of the high contributions of lakes water (e.g., Dongting Lake and Poyang Lake) efflux to the Yangtze River mainstream, which ranged from 21% to 85% during 2006 and 2013. These findings suggest that the retention and regulation of the Three Gorges Dams has greatly buffered the isotopic variability of the water cycle in the Yangtze catchment, thereby improving our understanding of the complex lake–river interactions along the middle to lower reaches in the future.     

The stable isotopes of natural waters at the Marcell Experimental Forest

We present a new data set from the Marcell Experimental Forest (MEF) that compiles water isotope measurements from multiple research catchments, some of which have been studied since the 1960s. The MEF is located in northern Minnesota, USA, and is home to heavily studied and monitored forests, streams, bogs, and fens. Peat-forming systems (bogs and fens) are an important component of the MEF landscape and have a profound impact on the water cycle in these catchments. Within the last decade, analysis of stable isotopes of water (expressed as δD and δ¹⁸O) has been implemented to characterize the different components of the water budget, and to allow researchers to look at catchment and peatland-specific hydrologic effects in the watershed. This δD and δ¹⁸O data set of natural waters from MEF catchments is primarily composed of measurements from three peatlands (S1, S2, S6) during an 11-year period. More recently collection and analysis were expanded to also include samples from the Spruce and Peatland Responses Under Changing Environments (SPRUCE) project in the S1 bog, peatlands S3, S4, S5, as well as nearby lakes. We establish a local meteoric water line by analyzing the isotopic composition of precipitation, which fills a void in regional meteoric water lines for Minnesota. Furthermore, we establish baseline isotopic composition for bog outlet streams, bog porewater, aquifer groundwater, overland flow, subsurface stormflow, and snowpack, as well as runoff from the SPRUCE experimental chambers. These data are publicly available and will be expanded upon in the future.     

Factors controlling stable oxygen,hydrogen and carbon isotope ratios in regional groundwater of the eastern United Arab Emirates (UAE)

Increasing groundwater salinity and depletion of the aquifers are major concerns in the UAE. Isotopes of oxygen, hydrogen, and carbon concentrations in groundwater were used to estimate evaporation loss using the isotopes of oxygen and hydrogen, and using a carbon isotope to trace inorganic carbon cycling in two main aquifers in the eastern part of the United Arab Emirates. The δD‐δ¹⁸O of groundwater samples plotted on a line given by: δD = 4 δ¹⁸O + 4 ·4 (r² = 0·4). In comparison, the local meteoric water line (LMWL) has been defined by the line: δD = 8 δ¹⁸O + 15. In order to better understand the system investigated, samples were separated into two groups based on the δD‐δ¹⁸O relationship. These are (1) samples that plot above the LMWL (δD = 6·1 δ¹⁸O + 12·4, r² = 0·8) and which are located predominantly in the north of the study area, and (2) samples that plot below the LMWL (δD = 5·6 δ¹⁸O + 6·2, r² = 0·8) and which are mostly distributed in the south. Slopes for both the groups are similar and lower than that for LMWL indicating potential evaporation of recharging water. However, the y‐intercept, which differs between the two groups, suggests evaporation of return flow and evapotranspiration in the unsaturated zone to be more significant in the south. This is attributed to intense agricultural activities in the region. Samples from the eastern Gravel Plain aquifer have δ¹³C and dissolved inorganic carbon (DIC) values in the range from ? 10 to 17‰, and 12–100 mg C/l, respectively, while the range for those from the Ophiolite aquifer is from ? 11 to ? 16.4‰, and 16–114 mg C/l respectively. This suggests the control of C‐3 and C‐4 plants on DIC formation, an observation supported by the range δ¹³C of soil organic matter (from ? 18·5 to ? 22·1‰.) Copyright © 2007 John Wiley & Sons, Ltd.     

An isotopic calibration study of precipitation,cave dripwater,and climate in west‐central Florida

A calibration study of oxygen and hydrogen isotopic composition from precipitation and cave dripwater was conducted in west‐central Florida at Legend Cave during 2007–2008. This study was performed to better understand how modern precipitation patterns can be discerned through examination of cave dripwater and speleothem calcite for paleoclimate reconstruction. The ‘amount effect’ was shown to be a dominant control on the oxygen isotopic composition of precipitation for the study area. A meteoric water line with a slope of 6·7 suggests evaporative effects occur either during precipitation or subsequent hydrological processes. However, δ¹⁸O values of cave dripwater averaged near the mean annual amount‐weighted average of precipitation, suggesting that the isotopic composition of dripwater tracks the long‐term average of rainfall. An observed weak seasonal influence occurred in the d‐excess values, with summer precipitation being more enriched due to increased evaporative effects. Comparison of precipitation δ¹⁸O values to synoptic weather data shows the dominant amount effect influence occurs due to strong convective storms producing highly ¹⁸O‐depleted rainfall at greater amounts during the year. Constant δ¹⁸O values of the dripwater indicate that paleoclimate reconstructions using speleothems from this area would record changes in annual to interannual shifts in precipitation amount above the cave. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of soil water hydrological process in the permafrost active layer using stable isotopes

Here, we studied the isotope characteristics and source contributions of soil water in the permafrost active layer by collecting soil samples in July 2018 in Yangtze River basin. Soil moisture and temperature showed decreasing trends from 0–80 cm, and an increasing trend from 80–100 cm. The value of δ¹⁸O and δD first increased and then decreased in the soil profile of 0–100 cm; however, d-excess increased from 0–100 cm. δ¹⁸O values became gradually positive from the southwest to northeast of the study area, while d-excess gradually increased from southeast to northwest. The evaporation water line (EL) was δD = 7.56 δ¹⁸O + 1.50 (R² = 0.90, p < 0.01, n = 96). Due to intense solar radiation and evaporation on the Tibetan Plateau, the elevation did not impact the surface soil. The altitude effect of the soil depths of 0–20 cm was not obvious, but the other soil layers had a significant altitude effect. Soil moisture and temperature were closely related to the stable isotopic composition of soil water. The contribution of precipitation to soil water on the sunny slope was 86%, while the contribution of the shady slope was 84%. However, the contribution of ground ice to soil water on sunny slope was 14% and the shady slope was 16%. The contribution of ground ice to soil water increased with increasing altitude on the sunny slope, but the contribution of ground ice to soil water had no obvious trend on the shady slope.     

Identifying the source of atmospheric moisture over arid deserts using stable isotopes (2H and 18O) in precipitation

Precipitation is a major component of the hydrologic cycle in arid desert areas. To date, however, few studies have been conducted on investigating the isotope characteristics and moisture sources of precipitation in arid desert environments. The Alxa Desert Plateau is a critical arid desert area in North China. This study is the first to analyse the stable isotopic composition of precipitation to identify the sources of atmospheric moisture over this plateau. Our results show that the δD and δ¹⁸O values of precipitation across the plateau change greatly at both daily and monthly timescales, and exhibit seasonal variations. Among the main meteorological parameters, atmospheric temperature is the most predominant factor controlling the isotopic composition and the δD–δ¹⁸O relationship of local precipitation. Analyses of the precipitation isotopes with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model reveal that (a) the westerly and polar moisture sources are the dominant controls on summer and winter precipitation and (b) the evaporation of local lake water significantly affects winter precipitation even though it only represents a small amount. Based on the isotope data of 2013–2016 precipitation, a local meteoric water line (LMWL) is derived: δD = (8.20 ± 0.22)·δ¹⁸O + (8.15 ± 2.16)‰ for the study site. Compared to the global meteoric water line, the LMWL has a greater slope and lower d‐excess. This can be explained by admixing of atmospheric moisture resulting from the evaporation of local lake water. Based on this LMWL, we are able to trace that groundwater of the Badain Jaran Desert originates from the surrounding mountains with altitudes of <4,000 m. The newly derived LMWL shows that the recharge altitudes of desert groundwater are overestimated on the basis of the previous LMWLs. This study not only provides insights into the hydrological cycle but also offers guidance for water resource management in arid desert areas of China. Additionally, this study provides techniques that can be applied to the analyses of precipitation isotopes in similar arid regions of the world.     

Oxygen and hydrogen isotope studies of serpentinization in the Troodos ophiolite complex,Cyprus

Oxygen and hydrogen isotopic compositions were measured on 12 serpentine and 2 actinolite samples from the Troodos ophiolite complex, Cyprus. The single analyzed antigorite(δD= ?60, δ¹⁸O= 7.1) is isotopically similar to all previously analyzed antigorites from ultramafic bodies. However, although their D/H ratios are relatively “normal”(δD= ?70to?92), the δ¹⁸O values of most of the Troodos lizardite-chrysotile serpentines (+12.6 to +14.1) are much higher than the 2.0–9.3‰ range typically found in such serpentines. Such high δ¹⁸O values have previously been found only in the serpentine-like mineraloids termed “deweylites”, which apparently formed at Earth-surface temperatures, and in a single sample from Vourinos, Greece that is in contact with high-¹⁸O limestone. The Troodos lizardite-chrysotile samplescannot have formed by reaction with heated ocean waters, but instead must have formed in contact with large amounts of some type of meteoric, metamorphic, or formation water, either (1) at very low temperatures in a near-surface environment, or (2) at about 100°C from waters that were abnormally enriched in¹⁸O(δ¹⁸O ≈ +4 to +8). The latter possibility seems most plausible inasmuch as extensive evaporites were deposited throughout the Mediterranean Sea during the late Miocene, and this would have been accompanied by strong¹⁸O enrichments of the local meteoric waters. Heated ocean waters, however, probably were responsible for the formation of the actinolitic amphiboles(δ¹⁸O= 4.6 to 5.5; δD= ?51to?46) from the gabbro and ultramafic zones in the Troodos complex. The amphiboles must have formed at considerably higher temperatures and at an earlier stage than the lizardite-chrysotile serpentinization.