Isotopic Composition Of Precipitation Research Articles

Moisture sources and isotopic composition of a record-breaking heavy Meiyu-Baiu rainfall in southwestern Japan in early July 2020

Torrential Meiyu-Baiu rainfall events have become more frequent in East Asia this decade. The moisture source and isotopic composition of precipitation are crucial for understanding the underlying dynamical and thermodynamic processes of extreme heavy Meiyu-Baiu rainfall. To clarify the moisture source and isotopic composition, we simulated a record-breaking heavy Meiyu-Baiu rainfall over Kyushu in southwestern Japan in early July 2020 using an isotopic regional spectral model. In addition, we observed the isotopic composition of the rainfall in Kyushu to validate the simulations. Moisture sources were classified into three groups: Asian monsoon (AM) moisture from the Indian Ocean, the South China Sea, and the East Asian Continent; North Pacific subtropical high (NPSH) moisture from the Pacific Ocean and the Philippine Sea; and local moisture from the East China Sea, the Kuroshio region, and the Sea of Japan. The remote AM and NPSH moisture was dominant. The NPSH moisture, transported by the NPSH at low levels, gradually increased in the early period. While AM moisture, transported by the monsoonal southwesterly flow at the mid-level, rapidly increased in association with a Baiu frontal depression and merged with the NPSH moisture in the middle period. Meanwhile, a quasi-stationary convective band (QSCB) formed and then triggered heavy rainfall. As for the isotopic composition, condensation exhibited lower δ2H and higher d-excess with more AM moisture in the beginning and on the north side of the QSCB. Regarding the effects of hydrometeorological processes on the isotopic composition, the lower δ2H and higher d-excess of AM moisture were principally attributable to more rainout and below-cloud evaporation during the transport process, rather than surface evaporation in moisture source regions. This study systematically clarified the major moisture sources and their corresponding isotopic dynamics with respect to the hydrometeorological processes of surface evaporation, moisture transport, rainout, and below-cloud evaporation for torrential Meiyu-Baiu rainfall. The findings may be useful in weather forecasting and disaster prevention for heavy Meiyu-Baiu rainfall in East Asia.

An open source Python library for environmental isotopic modelling

Isotopic composition modelling is a key aspect in many environmental studies. This work presents Isocompy, an open source Python library that estimates isotopic compositions through machine learning algorithms with user-defined variables. Isocompy includes dataset preprocessing, outlier detection, statistical analysis, feature selection, model validation and calibration and postprocessing. This tool has the flexibility to operate with discontinuous inputs in time and space. The automatic decision-making procedures are knitted in different stages of the algorithm, although it is possible to manually complete each step. The extensive output reports, figures and maps generated by Isocompy facilitate the comprehension of stable water isotope studies. The functionality of Isocompy is demonstrated with an application example involving the meteorological features and isotopic composition of precipitation in N Chile, which are compared with the results produced in previous studies. In essence, Isocompy offers an open source foundation for isotopic studies that ensures reproducible research in environmental fields.

Isotopic Composition of Precipitation in the Ten’kinskii District, the Magadan Oblast

Isotopic Composition of Precipitation in the Ten’kinskii District, the Magadan Oblast

Comparisons of Precipitation Isotopic Effects on Daily, Monthly and Annual Time Scales—A Case Study in the Subtropical Monsoon Region of Eastern China

The study on precipitation isotope variation can potentially improve the understanding of weather processes, regional water cycle and paleoclimate reconstruction in the subtropical monsoon region. Based on the measured stable isotope composition in precipitation (δ18Op) and daily precipitation from January 2010 to December 2021 in Changsha of the subtropical monsoon region of eastern China, the δ18Op variations, amount effect and local meteoric water line (LMWL) were analyzed and compared on daily, monthly and annual time scales, as well as under different precipitation intensities. The results showed that, on the daily time scale, δ18Op was significantly and negatively correlated with precipitation in the study area. Influenced by subcloud evaporation, small precipitation events (≤5 mm/d) could change the rainout level of precipitation isotopes. There were significant differences in the slope and intercept of the LMWL on different time scales, in different seasons and under different precipitation intensities. On the daily and monthly time scales, the slope and intercept of the LMWL in the cold half of the year were significantly smaller and larger than those in the warm half of the year, respectively, and the slope and intercept of the LMWL increased significantly with precipitation intensity, and then remained largely stable. On the annual time scale, the slope and intercept of the LMWL in the cold half of the year were smaller than those in the warm half of the year. The possible reasons for the differences in the LMWL on different time scales are the combined effects of seasonal differences in precipitation intensity and water vapor sources.

Predicting spatial distribution of stable isotopes in precipitation by classical geostatistical- and machine learning methods

Stable isotopes of precipitation are important natural tracers in hydrology, ecology, and forensics. The spatially explicit predictions of oxygen and hydrogen isotopes in precipitation are obtained through different interpolation techniques. In the present study we aim to examine the performance of various interpolation techniques when predicting the spatial distribution of precipitation stable isotopes. The efficiency of combined geostatistical tools (i.e. regression kriging; RK) and various machine learning methods (including regression enhanced random forest methods: MRRF, RERF) are compared in interpolating the spatial variability of precipitation stable oxygen isotope values from two different sampling networks in Europe. To assess the performance of the models, mean squared error (MSE), nonparametric Kling Gupta efficiency (KGE), absolute differences and relative mean absolute error metrics were employed. It was found that the combination of the different regression techniques with Random Forest can produce estimations with comparable accuracy in terms of descending order of overall average MSE, MRRF: 2.61, RK: 2.77, RERF: 2.99, RF: 3.08. The best performing combined random forest model variant (MRRF) outperformed regression kriging in terms of a hybrid error metric (KGE) by 7.5%. Sequential random rarefying the station networks showed that machine-learning methods are more capable of maintaining high prediction accuracy even with fewer input data. This can be a great advantage when a suitable method is needed to predict the stable isotope composition of precipitation for large spatial domains where the spatial density of data stations shows large differences.

Controls on the stable isotope composition of daily precipitation in Sydney Australia: 9 years of daily data, including Radon-222

Daily and weekly rainfall stable isotopic data, collected over a 9-year period (for daily) and 14-year period (for weekly) at Lucas Heights, Australia, were compared. Small differences in the local meteoric water lines (LMWLs) were seen when developed using daily, weekly, or monthly data (where the effect of short-term small atmospheric variability is reduced by the associated averaging). Although, if a precipitation weighted least squares method was used to derive the LMWLs, the differences between the daily and weekly derived LMWLs were lower.A systematic analysis was then conducted of the impact on the isotopic composition of precipitation from environmental conditions at the sampling site and along the air mass trajectory, the synoptic weather systems, and finally large-scale climate drivers (e.g.,El Niño-Southern Oscillation).The best correlation of δ18O was with local temperature, rainout of the air mass, and rainfall amount at the site. The correlation between precipitation amount and δ18O values was higher when daily samples were used, than when the precipitation weighted monthly values were considered (-0.36 as opposed to −0.31). A good correlation was also seen between δ18O and duration of the precipitation event (r = -0.42). The correlation between δ18O and the fraction of convective rainfall was significant, but low (r = 0.11).On a synoptic systems basis, the lowest δ values were seen in rainfall from offshore low-pressure systems, located to the east or north-east of the site; although, the largest rainfall was seen from low pressure systems arriving from the north-east. The influence of the large-scale climate drivers on the isotopic composition of precipitation was insignificant at this coastal site.When Radon-222 was used to categorise the atmospheric mixing state, rainfall amount was clearly negatively correlated with nocturnal stability, with the lowest amount of rainfall occurring on days with the most stable nocturnal periods. Most of the convective rainfall occurred on these same days, occurring after the warmer period of the day, although the total rainfall amount was low. Analysing the data within the separate radon-derived stability categories improved the correlations with rainfall amount for some of the stability categories.

Suspended in sound: New constraints on isotopic fractionation of falling hydrometeors using acoustically levitated water droplets

AbstractThe isotopic composition of precipitation (δ18O, δ2H, and δ17O) is affected by evaporation and exchange as hydrometeors descend. These processes can significantly alter the isotopic ratio of precipitation relative to its initial condensation state in the cloud yet are exceedingly difficult to study in situ. The most widely utilized model for droplet‐atmosphere exchange was derived from controlled experiments where droplets were suspended by forced air in a narrow glass tube‐ a design that manipulated the structure of the boundary layer around the droplet. Here, we provide a novel experimental test of atmosphere‐hydrometeor isotopic exchange using the mechanism of acoustic levitation, where sound waves are projected vertically to levitate droplets in free‐flowing ambient air. We present results from a series of droplet levitation experiments where the droplets' surface temperatures were measured by a thermal camera and the background atmospheric isotope concentration was measured via cavity‐ringdown spectroscopy, providing a high degree of constraint on the fractionation conditions. We show that isotope enrichment of the suspended droplets met first order expectations based on existing models. However, to account for the slope of δ18O versus δ2H (i.e., the meteoric water line) and deuterium excess of the droplets as they evolved, we had to modify the existing model for droplet‐atmosphere exchange to account for the fact that some portion of the evaporative flux from the droplet remained present in the boundary layer around the droplet leading to an evolving feedback between droplet and the atmosphere‐that is, a quasi closed‐system effect. The isotopic enrichment of the boundary layer surrounding the droplet as a consequence of the closed‐system dynamics, drives more rapid δ18O isotope enrichment relative to δ2H of the droplet compared to what is predicted using an open system model. Although these were controlled experiments, they illustrate important dynamics regarding the isotopic signature of feedbacks between droplet evaporation and atmospheric humidity.

Towards learned emulation of interannual water isotopologue variations in General Circulation Models

Abstract Simulating abundances of stable water isotopologues, that is, molecules differing in their isotopic composition, within climate models allows for comparisons with proxy data and, thus, for testing hypotheses about past climate and validating climate models under varying climatic conditions. However, many models are run without explicitly simulating water isotopologues. We investigate the possibility of replacing the explicit physics-based simulation of oxygen isotopic composition in precipitation using machine learning methods. These methods estimate isotopic composition at each time step for given fields of surface temperature and precipitation amount. We implement convolutional neural networks (CNNs) based on the successful UNet architecture and test whether a spherical network architecture outperforms the naive approach of treating Earth’s latitude-longitude grid as a flat image. Conducting a case study on a last millennium run with the iHadCM3 climate model, we find that roughly 40% of the temporal variance in the isotopic composition is explained by the emulations on interannual and monthly timescale, with spatially varying emulation quality. The tested CNNs outperform simple baseline models such as random forest and pixel-wise linear regression substantially. A modified version of the standard UNet architecture for flat images yields results that are as good as the predictions by the spherical CNN. Variations in the implementation of isotopes between climate models likely contribute to an observed deterioration of emulation results when testing on data obtained from different climate models than the one used for training. Future work toward stable water-isotope emulation might focus on achieving robust climate–oxygen isotope relationships or exploring the set of possible predictor variables.

Estimation of stream water components and residence time in a permafrost catchment in the central Tibetan Plateau using long-term water stable isotopic data

Abstract. Global warming has significantly impacted the hydrological processes and ecological environment in permafrost regions. Mean residence time (MRT) is a fundamental catchment descriptor that provides hydrological information regarding storage, flow pathways, and water source within a particular catchment. However, water stable isotopes and MRT have rarely been investigated due to limited data collection in the high-altitude permafrost regions. This study uses the long-term stable isotopic observations to identify runoff components and applied the sine-wave exponential model to estimate water MRT in a high-altitude permafrost catchment (5300 m a.s.l.) in the central Tibetan Plateau (TP). We found that the isotope composition in precipitation, stream, and supra-permafrost water exhibited obvious seasonal variability. The freeze–thaw process of the permafrost active layer and direct input of precipitation significantly modified the stable isotope compositions in supra-permafrost and stream water. The hydrograph separation revealed that precipitation and supra-permafrost water accounted for 35 ± 2 % and 65 ± 2 % of the total discharge of stream water, respectively. MRT for stream and supra-permafrost water was estimated at 100 and 255 d, respectively. Such shorter MRTs of supra-permafrost and stream water (compared to the non-permafrost catchments) might reflect the unique characteristics of the hydrological process in permafrost catchments. Moreover, the MRT of supra-permafrost water was more sensitive to environmental change than that of stream water. Climate and vegetation factors affected the MRT of stream and supra-permafrost water mainly by changing the thickness of the permafrost active layer. Our results suggest that climate warming might retard the rate of water cycle in permafrost regions. Overall, our study expands our understanding of hydrological processes in high-altitude permafrost catchments under global warming.

Isotope composition of daily precipitation from 2019 to 2020 in Sanming, southeastern China

Introduction: Many studies in southeastern China (SEC) have used stable isotope (δD and δ18O) records from natural archives (e.g., stalagmites, tree-ring cellulose, sediments) to reconstruct past East Asian Summer Monsoon (EASM) climate. However, the interpretation of the meanings of isotope variation in these records has not been fully resolved, and the key to solve this problem is to clarify the referential meaning of modern precipitation isotopes.Methods: In this study, we collected daily precipitation from January 2019 to December 2020 in Sanming to clarify the characteristics of isotope variations and their controlling factors during different seasonal periods [e.g., non-summer monsoon (NSM) and summer monsoon (SM)] in SEC.Results and Discussion: Our results show that the precipitation δ18O (δ18Op) and d-excess values in the SM season (−12.94‰–0.15‰, −4.05‰–21.01‰) were more light than those in the NSM season (−6.91‰–4.37‰, 0.85‰–30.38‰). Combining the findings of backward trajectory and averaged outgoing longwave radiation analyses, the seasonal variation of precipitation isotopes is believed to be determined by a shift in water vapor sources and the intensity of upstream atmospheric convection during water vapor transportation. The consistent variation between the isotope values and convective intensity over the South China Sea in the dragon boat rainy period highlights that δ18Op has the potential to respond rapidly to the upstream convective intensity and can serve as a substitute method for investigating the complicated East Asian summer monsoon system. Therefore, the results of this study imply that the stable isotopes in precipitation and related paleoclimate proxies may not reflect the signal of temperature or precipitation alone but rather reflect changes in moisture sources and upstream convective intensity.

Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

AbstractWe evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (δ2Hp) and oxygen (δ18Op) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (δ2Hwax) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. δ2Hp, δ18Op, as well as the unweighted mean δ2Hwax values of n‐C29, n‐C31, and n‐C33 n‐alkanes in the eastern flank show a dependence on elevation (R2 = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R2 < 0.23), on top of a negligible (p‐value >0.05) correlation between δ2Hwax and δ2Hp. In general, δ2Hwax values along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither δ2Hp nor δ18Op, and therefore δ2Hwax, offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, δ2Hwax is only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on δ2Hp and/or δ18Op west of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount.

What Controls the Skill of General Circulation Models to Simulate the Seasonal Cycle in Water Isotopic Composition in the Tibetan Plateau Region?

AbstractThis study evaluates the simulation of the seasonal cycle of water isotopic composition over Tibetan Plateau regions (TP) from six isotope‐enabled general circulation models (GCMs) participating in the second Phase of Stable Water Isotope Intercomparison Group. For both meteorological factors (precipitation rate and wind field) and isotopic composition, GCMs generally agree with reanalysis data and in‐situ observations, but there is a significant spread across models and the isotopic seasonality is systematically underestimated. In the southern TP, the precipitation isotopic composition is more depleted in summer than in winter, and the amplitude of the simulated isotopic seasonal variations is primarily driven by the amplitude of the simulated upstream precipitation. In contrast, in the northern TP, the precipitation isotopic composition is more depleted in winter than in summer, and the amplitude of the simulated seasonal variability of isotopes is mainly driven by the simulated strength of the zonal wind. We conclude that the skill of a GCM to simulate the seasonal cycle in the isotopic composition depends mainly on the skill of the GCM to simulate the Indian summer monsoon precipitation and the westerlies. The same causes contributing to the underestimated seasonality at present‐day may also contribute to the underestimated δ18O change at the mid‐Holocene.

Disentangling the Contribution of Moisture Source Change to Isotopic Proxy Signatures: Deuterium Tracing with WRF-Hydro-Iso-Tag and Application to Southern African Holocene Sediment Archives

Abstract It is well accepted that global circulation models equipped with stable water isotopologues help us better understand the relationships between atmospheric circulation changes and isotope records in paleoclimate archives. Still, isotope-enabled models do not disentangle the different processes affecting precipitation isotopic compositions. Furthermore, the relevance of this model-oriented approach relies on the realism of the modeled isotope results, which would support the interpretation of the proxy records in terms of modeled climate changes. To alleviate these limitations, the newly developed WRF-Hydro-iso-tag, a version of the isotope-enabled regional coupled model WRF-Hydro-iso enhanced with an isotope-tracing procedure, is presented. Physics-based WRF-Hydro-iso-tag ensembles are used to regionally downscale the isotope-enabled Community Earth System Model for southern Africa, for two 10-yr slices of mid-Holocene and preindustrial times. The isotope-tracing procedure is tailored to assess the origin of the hydrogen isotope deuterium contained in southern African precipitation, between the Atlantic and Indian Oceans. In comparison to the global model, WRF-Hydro-iso-tag simulates lower precipitation amounts with more regional details, as well as mid-Holocene-to-preindustrial changes in precipitation isotopic compositions that better match plant-wax deuterium records from two marine sediment cores off the Orange and Limpopo River basins. Linear relationships between mid-Holocene-to-preindustrial changes in temperature, precipitation amount, moisture source, and precipitation deuterium compositions are derived from the ensemble results. A deuterium enrichment in the Orange River-related sediment core may not be related to an aridification but rather indicate a summer circulation change enabling a larger contribution of more isotopically enriched moisture from the Atlantic Ocean. Significance Statement The knowledge of past climates is crucial for understanding our Earth system and apprehending future climate change. Plant materials in sediment archives contain atoms of hydrogen from past precipitation that allow paleoclimate reconstructions, using compositions of the hydrogen isotope deuterium. However, in the tropics, deuterium-depleted plant remains can either denote a wetter climate phase or a change in atmospheric circulation patterns with longer distances between ocean evaporation and land precipitation. This work provides an innovative dynamical downscaling method of global paleoclimate models to disentangle the effects of precipitation amount change and moisture source change on deuterium records, and ultimately to improve paleoclimate reconstructions. The interpretation of a deuterium enrichment in a marine sediment core as a marker for aridification is revised. The enrichment caused by an atmospheric circulation change bringing a larger amount of more isotopically enriched moisture flow from the Atlantic Ocean to southern African precipitation would be a more physically sound explanation.

Younger Dryas and early Holocene climate in south Greenland inferred from oxygen isotopes of chironomids, aquatic Moss, and Moss cellulose

Ice core records have long indicated that the Younger Dryas began and ended with large, abrupt climate shifts over Greenland. Key climatic features remain unknown, including the magnitude of warming during the Younger Dryas-Holocene transition along with the seasonality and spatial variability of Younger Dryas climate changes across Greenland. Here, we use geochemical and paleoecological proxies from lake sediments at Lake N14 in south Greenland to address these outstanding questions. Radiocarbon dating and diatom assemblages confirm early deglaciation and isolation of Lake N14 before ∼13,600 cal yr BP, consistent with previous work. Oxygen isotope ratios (δ18O) of chironomid head capsules, bulk aquatic moss, and aquatic moss-derived cellulose are used to reconstruct oxygen isotopes of past lake water and annual precipitation. Oxygen isotope proxies indicate annual precipitation δ18O values increased by 5.9–7.7‰ at the end of the Younger Dryas. Following the Younger Dryas, moss and cellulose δ18O values show a clear decline in precipitation δ18O values of 2–3‰ from ∼11,540–11,340 cal yr BP that may correspond with the Preboreal Oscillation. Reconstructed precipitation δ18O values then gradually increased from 11,300–10,100 cal yr BP. All three aquatic organic materials register similar shifts in precipitation δ18O values over time, and they closely parallel the δ18O shifts observed in ice cores. This evidence strongly supports the utility of these methods for reconstructing lake water δ18O, and furthermore precipitation δ18O values where lake water reflects precipitation. The relatively large shift in isotopic composition of precipitation at Lake N14 suggests that shifts in temperature, precipitation seasonality, and/or moisture sources at the end of the Younger Dryas were even larger in south Greenland than they were in central Greenland, most likely because of the proximity to major changes in North Atlantic Ocean circulation. The annual air temperature change estimated at Lake N14 at the end of the Younger Dryas is also very large (∼18 ± 7 °C) compared to the summer warming previously inferred from chironomid species assemblages there (∼6 °C). This indicates that the strongest warming at the end of the Younger Dryas occurred in the winter season, consistent with past observations of intensified Younger Dryas seasonality at Lake N14 and elsewhere in Greenland.

Conditions of groundwater recharge in the hyperarid southern Atacama Desert

Weather systems that produced recharge to the aquifers in the Atacama Desert between 24.5 and 25.5°S were investigated using δ 18 O and δ 2 H data in groundwater and precipitation combined with remote sensing methods. Atmospheric systems that produced rain and snow from 1984 to 2017 were analysed using isotopic data for precipitation from the literature and this investigation. δ 18 O-lapse rates on a west to east transect for a series of precipitation events ranged from −5.0 to −1.1 ‰/km, which is similar to the global average. Groundwater stable isotopes plotted against the average elevation of the catchment basin for each spring or well sampled produce a lapse rate identical to precipitation isotopes but are enriched in δ 18 O relative to δ 2 H due to the effects of evaporation before water enters the groundwater system. Data show a seasonal cycle in the isotopic composition of precipitation, as does δ 2 H data obtained from Tropospheric Emission Spectrometer (TES) on-board the Aura satellite. These analyses demonstrate that an important source of moisture is the Pacific Ocean. Although usually discounted in climate and paleoclimate analyses of northern Chile, this source of moisture is consistent with two features of the regional land-atmosphere system. First, the hyper-arid core of the Atacama Desert at these latitudes lies in a secondary rain shadow created by the Cordillera de Domeyko which exceeds 4000 m a.s.l., which blocks further westward passage of moisture that might have crossed from the east over the Andes. Second, the weather events responsible for precipitation between 2000 and 2018 are distributed between winter systems which initiate from the south, and exceptional events at other times of the year in which moisture originates from the tropics. Particle tracking indicates that air masses resided between 10 and 30°S over the Pacific at least 4 days prior to weather events which produced rain, or snow that persisted on the ground for more than one month between 2000 and 2018, even though the triggers for these events originated in higher latitudes. Pacific moisture sources need to be better accounted for in paleoclimate studies and groundwater resources evaluation. • Precipitation collected in the southern part of the Atacama Desert is isotopically characterized. • Seasonal patterns are found in the isotope composition of precipitation and atmospheric water vapor. • Isotope data indicate moisture is derived from the Pacific Ocean, and is confirmed from particle tracking. • Groundwater isotope data indicates moisture has been derived from the Pacific Ocean on millennial timescales.

Last millennium hydroclimate and atmospheric circulation change in Northeast China: A dual δ13C and δ18O approach from a mountaintop Sphagnum bog

Northeast China receives most moisture through warm air masses from the western Pacific Ocean and some moisture from the Eurasian Continent predominantly through the westerlies. Changes in the contribution of these different moisture sources affect the amount and isotopic composition of precipitation, which would in turn influence peatland surface moisture and peat moss (Sphagnum) growth. Recently a dual δ13C and δ18O approach from peat-core archive has been used not only as a proxy for peatland moisture/wetness but also as a constraint for interpreting isotope in precipitation and moisture trajectories. Here we used the coupled δ13C and δ18O measurements of peat moss cellulose, along with macrofossil and other proxy data, from a well-dated peat core retrieved from a Sphagnum-dominated ombrotrophic (meteoric water-fed) bog in Northeast China to reconstruct regional climate and moisture sources of precipitation over the last millennium.Our results show no sign of extremely negative δ13C signals that would have been caused by the uptake of respired CO2 by peat moss for photosynthesis, thus we argue that the cellulose δ13C signatures document the peatland moisture conditions, owing to water-film effect. We found a positive correlation between δ13C and δ18O, suggesting that δ18O primarily reflects the δ18O signal in precipitation rather than evaporative 18O-enrichment, because the evaporative enrichment would result in a negative correlation between these isotopes as documented in modern process studies along the hummock-hollow moisture gradients. We propose that the positive δ13C-δ18O correlation is a general feature in regions that are influenced by both summer monsoon and the westerlies, as warm Pacific moisture that has high δ18O values also brings abundant precipitation, resulting in wet conditions that induce high δ13C values, and vice versa. The δ13C results show a decrease of ca. 6‰ from −23 to −29‰, with large-magnitude fluctuations, over the last 1000 years, suggesting a long-term drying trend. An abrupt decrease at 1370 CE of 4‰ in δ18O from >21‰ to ca. 17‰ represents the transition from the warm and wet Medieval Climate Anomaly (MCA) at 815–1370 CE to the cold and dry Little Ice Age (LIA) at 1370–1940 CE, reaching the lowest δ18O value of ca. 15‰ at around 1850 CE. The increase in δ18O to 19‰ after 1900 CE reflects post-LIA climate warming. Enhanced moisture transport from the western Pacific might have contributed to the warm and wet MCA. Three short intervals with pronounced low δ13C and δ18O values occurred at 1370–1450 CE, the 19th Century, and the 1990s, suggesting extreme dry periods with reduced Pacific moisture inputs. The driest period with consistently low δ13C value of < -25‰ (Suess effect-corrected value of −23‰) occurred after 1990 CE, with a corresponding increase in dry-adapted moss Polytrichum.Variability in the regional atmospheric circulation—the western Pacific subtropical high (WPSH)—might have explained the change in moisture sources. The westward extension of the WPSH—often corresponding with the El Niño-like conditions—would block the Pacific moisture from transporting to Northeast China. Our record, together with other paleo-records over monsoon-influenced regions in East Asia, shows synchronous but opposite shifts of summer rainfall anomalies in North or Northeast China and South China on decadal to centennial scales, substantiating a coherent atmospheric circulation pattern associated with WPSH and ENSO variability. We argue that there were major shifts in synoptic-scale atmospheric circulation and associated hydroclimate change during the MCA, LIA, and Current Warming Period in Northeast China.

Stable isotope composition in precipitation and groundwater of Shwan Sub-Basin, Kirkuk governorate, northeast of Iraq

Abstract Stable isotope composition of δ2H and δ18O was investigated in the water resources of the Shwan sub-Basin northeast of Iraq. The study objects conceived the possible factors that affect the stable isotopes’ composition in precipitation additionally to achieve information concerning recharge processes and estimate the groundwater recharge sources. In this study, four precipitation samples were collected at the study area for the 2020–2021 hydrological year. Thirty-two groundwater samples and one surface water sample from Lesser Zab River (LZR) were collected during the same period for two sampling seasons. The results of observed meteorological data show a very small amount of precipitation for the sampling year. This year is considered a dry hydrological year with total annual precipitation of 100.62 mm compared with the previous 40 hydrological years with total annual precipitation of 325.43 mm. The isotopic composition in precipitation was highly varied as it primarily depends on environmental conditions. The depleted values are recognized with the increasing precipitation amount, whereas the enriched values were the most affected by evaporation. Back trajectory analysis revealed that stable isotopes in precipitation are primarily influenced by air masses and moisture sources. The sources of the trajectory that came from the Mediterranean Sea, Arabian Gulf and the Red Sea would lead to variations in the values of precipitation stable isotope. Stable isotope values in groundwater showed that the samples for both periods are located between the East Mediterranean water line (EMWL) and global meteoric water line (GMWL) close to the local meteoric water line (LMWL) indicating that the groundwater recharge is mainly through precipitation. Groundwater recharges by an indirect recharge mechanism from the LZR, based on stable isotope similarity between depleted stable isotopes in groundwater and river water. The estimated groundwater recharge based on weighted oxygen isotopes is about 9.2% of annual rainfall infers that the recharge during the sampling year was very low. The low recharge value experiences dry weather conditions from low precipitation amounts besides increasing evaporation during the current study.

TRITIUM CONCENTRATION IN MONTHLY PRECIPITATION NEAR THE FUSION TEST FACILITY IN JAPAN BEFORE AND AFTER THE DEUTERIUM PLASMA EXPERIMENT.

In Japan, the deuterium plasma experiment using the Large Helical Device was started at the National Institute for Fusion Science (NIFS) in March 2017 to investigate high-temperature plasma physics and hydrogen isotope effects in research leading towards the realisation of fusion energy. The deuterium plasma experiment produces small amount of tritium by fusion reactions. To understand any impacts by the experiment to the surrounding environment, monthly precipitation samples have been collected at the NIFS site since November 2013 to assess the relationship between isotope composition and chemical species in precipitation including tritium. By comparing data before and after the deuterium plasma experiment start, it was found that tritium released from the main stack of the fusion test facility had no impact on the environment surrounding NIFS.

Seasonal isotopic cycles used to identify transit times and the young water fraction within the critical zone in a subtropical catchment in China

Transit times and the young water fraction (Fyw, the proportion of the water younger than a certain age threshold, τyw) can help characterize the hydrological behaviors within a catchment. Previous studies of water age have concentrated on individual catchment hydrological components, such as river water, groundwater, plant water or soil water, which hinders the dynamic linking of various hydrological components in the critical zone. In this study, the isotopic compositions of precipitation, soil water, xylem water, river water, and groundwater in a subtropical catchment in China were continuously monitored for two years. We found that the seasonal isotopic cycles of shallow soil water (0–60 cm), xylem water and river water had different attenuations and phase shifts relative to those of precipitation. There were no obvious seasonal isotopic cycles of deep soil water (60–130 cm) and groundwater. Additionally, the transit time distributions (TTDs) of soil water at the depth of 0–10 cm, xylem water and river water in the catchment varied between the wet year and normal year. With increasing depth, the mean transit times (MTTs) of soil water increased and the Fyw decreased. For xylem water and soil water, the MTTs and Fyw in the wet year were shorter and larger than those in the normal year, respectively. The river water consisted of a small proportion of fast flow components and a large proportion of slow flow components, with the young water accounting for 16% in the wet year and 21% in the normal year. Our results contribute to an improved understanding of water ages, flow paths and linkages of different hydrological components in the critical zone at the catchment scale.

Diurnal Impact of Below-Cloud Evaporation on Isotope Compositions of Precipitation on the Southern Slope of the Altai Mountains, Central Asia

Precipitation is an important natural resource relating to regional sustainability in arid central Asia, and the stable oxygen and hydrogen isotopes provide useful tracers to understand precipitation processes. In this study, we collected the hourly meteorological data at several stations on the southern slope of the Altai Mountains in arid central Asia, from March 2017 to June 2022, and examined the diurnal impact of below-cloud evaporation on stable isotope compositions of precipitation. During nighttime, the changes in isotope compositions below cloud base are generally weak. The enhanced impact of below-cloud evaporation can be found after around 15:00, and the impact is relatively strong in the afternoon, especially from 18:00 to 22:00. Summer and spring usually have a larger impact of below-cloud evaporation than autumn, and the winter precipitation is generally not influenced by below-cloud evaporation. On an annual basis, the differences in evaporation-led isotope changes between daytime and nighttime are 1.1‰ for stable oxygen isotope compositions, 4.0‰ for stable hydrogen isotope compositions and 4.7‰ for deuterium excess. The period from 2:00 to 10:00 shows relatively low sensitivity to relative humidity, and from 14:00 to 22:00 the impacts are sensitive. Considering the fluctuations of precipitation isotope compositions, the impact of below-cloud evaporation does not greatly modify the seasonal environmental signals.