Water Isotopic Signatures Research Articles

Interaction regimes of surface water and groundwater in a hyper-arid endorheic watershed on Tibetan Plateau: Insights from multi-proxy data

The interaction between surface water and groundwater is crucial for the management of water resources and the preservation of ecosystems within arid basins, but knowledge on their interaction regimes at the watershed scale remains relatively limited. The present research synthesizes a range of multi-proxy data, including satellite thermal infrared remote sensing, water piezometric level, hydrochemical analyses, and isotopic signatures (2H, 18O and 222Rn), to elucidate the interaction dynamics and patterns between surface water and groundwater within a representative hyper-arid closed basin on Tibetan Plateau. The findings indicate that the water in the basin originates primarily from the glacier/snow melt water and precipitation in the mountainous regions, and would experience multiple but spatially heterogeneous interactions between river and aquifers from the mountain pass to the tail salt lakes after entering the basin. Water interaction in the piedmont alluvial plain is dominated by the pattern of river seepage into aquifers with a water flux of 25.82 m3/s, which drives the development of hierarchical groundwater flow systems in the basin. The interaction pattern converts to groundwater discharge of the local groundwater flow system at the front of alluvial fan plain, which forms the principal spring-fed rivers and the predominant overflow zone in the basin with the groundwater discharge flux of 3.22 m3/s. Most of these discharged groundwaters would infiltrate back into the aquifers in the middle-upper part of the loess plain with the river water leakage flux of 2.89 m3/s. River water and phreatic groundwater present a gradual enrichment of hydrochemcial components and water stable isotopes (2H and 18O) along the flow path due to the intense evaporation in the loess plain. The multiple water interactions between surface water and groundwater lead to the anomalous distribution of fresh water in the middle-lower stream area, which is related to the intermediate groundwater flow system and the local variable groundwater flow system. Water interaction in the lowest salt-marsh plain is in the pattern of regional groundwater flow system discharge into the tail salt lakes under natural condition, but evolves to only discharge into the artificial brine extraction canals rather than the tail salt lakes after decades of brine exploitation. Our findings provide a conceptual and preliminarily quantificational understanding of the interaction regimes between surface water and groundwater in large arid closed basins at the watershed scale.

Employing stable isotopes to reveal temporal trajectories of water travelling through the soil–plant-atmosphere continuum

The spatiotemporal patterns of water travelling through the soil–plant-atmosphere continuum (SPAC) have received much attention due to the frequency of extreme weather and water scarcity. However, the interconnections and transboundary transport of specific compartments within the hydrologic cycle are poorly understood. We portrayed the propagation paths of water isotope signals by analyzing isotopes of precipitation, soil water, and xylem water. The isotope sine curves analysis method was improved to quantify water transfer efficiency, mean transmission time from precipitation to soil (MTT1), mean transmission time from soil to trees (MTT2), mean residence time within soil (MRTSW), and mean residence time within tree xylems (MRTXY). The temporal trajectory of water traveling through SPAC was depicted, and its influencing factors and intrinsic mechanisms were explored. Our results showed that (1) water isotopes were blocked when crossing water pools (precipitation, soil water, and xylem water) and the transfer efficiency was progressively weaker (85.5 %→13.5 %). (2) The MTT1 was significantly and positively correlated with soil porosity (Spearman correlation coefficient, 0.551). Its spatiotemporal pattern (0.8–15.9d) indicated that the transport and mixing of soil water involved two modes: displacement flow and bypass flow. The MTT2 (−13.2d–4.3d) of Platycadus orientalis was in general smaller than that of Quercus variabilis (−4.7d–11.5d). (3) The MRT (35.2d–343.8d) was influenced by a combination of soil physicochemical properties, root distribution, and tree morphological traits. Our study shows that connectivity between pools is better reflected with transfer efficiency. Comparing MTT and MRT of P. orientalis and Q. variabilis, tree hydrodynamic processes were quantified, and P. orientalis was more sensitive to seasonal moisture variability. Additionally, our study improved the understanding of the “black box” within the hydrological interface of plant-soil interactions.

Stable Water Isotope Signals and Their Relation to Stratiform and Convective Precipitation in the Tropical Andes

AbstractStratiform and convective precipitation are known to be associated with distinct isotopic fingerprints in the tropics. Such rain type specific isotope signals are of key importance for climate reconstructions derived from climate proxies (e.g., stable isotopes in tree rings). Recently, the relation between rain type and isotope signal in present‐day climate has been intensively discussed. While some studies point out the importance of deep convection, other studies emphasize the role of stratiform precipitation for strongly depleted isotope signals in precipitation. Uncertainties arise from observational studies due to data scarcity while modeling approaches with global climate models cannot explicitly resolve convective processes and rely on parameterizations. High‐resolution climate models are particularly important for studies over complex topography and for the simulation of convective cloud formation and organization. Therefore, we applied the isotope‐enabled version of the high‐resolution climate model from the Consortium for Small‐Scale Modeling (COSMOiso) over the Andes of tropical south Ecuador, South America, to investigate the influence of stratiform and convective rain on the stable oxygen isotope signal of precipitation (δ18OP). Our results highlight the importance of deep convection for depleting the isotopic signal of precipitation and increasing its deuterium excess. Due to the opposing effect of shallow and deep convection on the δ18OP signal, the use of a stratiform fraction might be misleading. We therefore propose to use a shallow and deep convective fraction to analyze the effect of rain types on δ18OP.

The preservation of photosynthetic and hydrological signals in the carbon and hydrogen isotope compositions of n-fatty acids in the seasonal wetland soils of the Okavango Delta (Botswana)

The Okavango wetland (Botswana) is the world’s largest inland delta. A strong seasonality in water input leads to the contraction and extension of wetlands in the floodplains. The extreme evapotranspiration and little precipitation lead to a difference in the hydrogen isotope signature of rain, soil and river water. Biomarkers, such as plant waxes, are stored in the soils and preserved on geological timescales. To understand which signal is preserved in the stable isotope signatures of plant waxes, soils along a 250 m-long transect spanning waterlogged to dry soils were collected over several seasons and three years. In addition, plants, and plant and soil water were collected along this transect. First, carbon isotope ratios (δ13C) of plant waxes (i.e, n-fatty acids) were used to classify their metabolism. δ13C of bulk organic matter and individual n-fatty acids analyzed in the soils show a strong dependance on the type of vegetation found along the transect (C3 versus C4 plants). Hydrogen isotope ratios (δ2H) of water present in soil showed that shallow-rooted C4 grasses use superficial soil water, whereas the xylem water δ2H content in trees growing near the flooded channel indicated the use of river water. In addition, plant hydrogen fractionation between lipids and rain showed a strong influence of carbon metabolisms with larger fractionation for C3 plants compared with C4 grasses. n-fatty acid δ2H ratios in surface soils followed the hydrological variation in the Delta with its floods and dry periods. Hence δ2H of long-chain fatty acids seems to track the river-level variation rather than precipitation.

Exploring Ice Cave Biodiversity in Northeastern Italy

The ice stored in caves is a widespread yet neglected cryospheric component. The cold-adapted biodiversity of ice caves has received very little attention from research, despite the potential abundance of endemic troglobiotic and cryophilic species and their consequent sensitivity to the changing climate. In this study, we investigated the invertebrate diversity of two ice caves in Northeastern Italy (Bus delle Taccole and Caverna del Sieson, Veneto Region). During 2022 and 2023, we sampled, using pitfall traps, the invertebrates dwelling at different locations in each cave: the shaft base, an intermediate hall, and the cave bottom. At each cave location, we also collected ice samples, on which we measured the stable isotopes of oxygen and hydrogen (δ18O, δ2H), and monitored the air temperature with data-loggers. The two caves had different invertebrate communities, both dominated by a combination of troglobiotic and cryophilic taxa. Despite a low taxonomic richness, which was higher at Taccole (15 taxa) than at Sieson (11 taxa), both caves hosted rare/endemic species, four of which are not described yet. At each cave, the ice water isotopic signatures differed among cave locations, suggesting the ice had formed under different climatic conditions, and/or resulted from different frequencies of thawing/freezing events. The occurrence of summer melt at both caves suggests that these unique ecosystems will quickly disappear, along with their specialized and unique biodiversity.

Review article: Melt-affected ice cores for polar research in a warming world

Abstract. Melting polar and alpine ice sheets in response to global warming pose ecological and societal risks but will also hamper our ability to reconstruct past climate and atmospheric composition across the globe. Since polar ice caps are crucial environmental archives but highly sensitive to ongoing climate warming, the Arctic and Antarctic research community is increasingly faced with melt-affected ice cores, which are already common in alpine settings of the lower latitudes. Here, we review the characteristics and effects of near-surface melting on ice-core records, focusing on a polar readership and making recommendations for melt-prone study regions. This review first covers melt layer formation, identification and quantification of melt, and structural characteristics of melt features. Subsequently, it discusses effects of melting on records of chemical impurities, i.e. major ions, trace elements, black carbon, and organic species as well as stable water isotopic signatures, gas records, and applications of melt layers as environmental proxies. Melting occurs during positive surface energy balance events, which are shaped by global to local meteorological forcing, regional orography, glacier surface conditions and subsurface characteristics. Meltwater flow ranges from homogeneous wetting to spatially heterogeneous preferential flow paths and is determined by temperature, thermal conductivity and stratigraphy of the snowpack. Melt layers and lenses are the most common consequent features in ice cores and are usually recorded manually or using line scanning. Chemical ice-core proxy records of water-soluble species are generally less preserved than insoluble particles such as black carbon or mineral dust due to their strong elution behaviour during percolation. However, high solubility in ice as observed for ions like F−, Cl−, NH4+ or ultra-trace elements can counteract the high mobility of these species due to burial in the ice interior. Stable water isotope records like δ18O are often preserved but appear smoothed if significant amounts of meltwater are involved. Melt-affected ice cores are further faced with questions about the permeability of the firn column for gas movement, and gas concentrations can be increased through dissolution and in situ production. Noble gas ratios can be useful tools for identifying melt-affected profile sections in deep ice. Despite challenges for ice-core climate reconstruction based on chemical records, melt layers are a proxy of warm temperatures above freezing, which is most sensitive in the dry snow and percolation zone. Bringing together insights from snow physics, firn hydrology, and ice-core proxy research, we aim to foster a more comprehensive understanding of ice cores as climate and environmental archives, provide a reference on how to approach melt-affected records, and raise awareness of the limitations and potential of melt layers in ice cores.

Saline and hydrocarbon-bearing fluids detected in shallow aquifers of southern New Brunswick, Canada: Natural occurrence, or deep migration along faults and industrial wellbores?

Unconventional hydrocarbon production has sparked public concerns for several years, especially regarding potential potable groundwater contamination by hydrocarbons, brines, and various chemicals related to hydraulic fracturing operations. One possible contamination mechanism is upward migration of deep-seated contaminants over large vertical distances, through preferential pathways such as leaky well casings or permeable geological faults. In New Brunswick (Canada), thermogenic hydrocarbons and brackish water were previously reported in shallow water wells, some of them located close to unconventional gas wells or to major faults, but the exact origin of these fluids remained uncertain. The objective of this paper is to determine whether the presence of these fluids is the result of migration from the deep (>1 km) hydrocarbon bearing units (via natural or anthropogenic migration pathways), or whether they rather originate within the shallow aquifer (<100 m) or from intermediate zone. Tracking fluid origin was achieved by fingerprinting compositional and isotopic values of three indicators: 1) water isotopic signature (including tritium (3H), radiocarbon (14CDIC), δ18OH2O, δ2HH2O), 2) salinity (including Na, Ca, K, SO4, Cl, Br, 87Sr/86Sr), and 3) hydrocarbons (compositional data and δ13CCH4). These various analyses were conducted, when relevant, on samples of different matrices composing the hydrogeological system, namely shallow groundwater (12–90 m depth), shallow bedrock gas (8–131 m), and intermediate zone evaporitic rocks (173–332 m); they were compared with previously published values for deep basin brines and gases (1940–3168 m) from the hydrocarbon bearing Carboniferous Albert Formation. This unique suite of indicators, analytes and matrices allowed drawing the conclusion that thermogenic gas and high salinities present in the sampled wells were naturally occurring and originating from shallow and intermediate-zone bedrock units. Results obtained through this approach did not provide any evidence that hydrocarbon wells in this area have acted as preferential migration pathways for deep-seated fluids towards shallow aquifers.

Tree‐ and stand‐scale variability of xylem water stable isotope signatures in mature beech, oak and spruce

AbstractIn ecohydrology, water isotopologues are used to assess potential sources of root water uptake by comparing xylem water signatures with source water signatures. Such comparisons are affected by the variability and uncertainty of the isotope signatures of plant water and water sources. The tree‐scale and stand‐scale variabilities of the isotope signatures in stem xylem water are often unknown but are important for sampling design and uncertainty estimation in assessing the sources of tree water uptake. Here, we quantified tree‐scale and stand‐scale variabilities of xylem water isotope signatures in beech, oak and spruce trees in a mature forest on the Swiss plateau. For stem xylem water, sub‐daily replicates and replicates in different cardinal directions showed no systematic differences, but we found systematic differences with sampling height. The observed variability of isotope signatures at different heights along the stem suggests that water residence times within trees need to be considered, along with their effects on the isotope signatures in different compartments (stem, branches, leaves). Further, concerning the hydrogen signatures, we found height‐ and species‐specific offsets (SW‐excess δ2H). Stem xylem water's tree‐scale variability was similar in magnitude to its stand‐scale variability and smaller than the variabilities in branch xylem and bulk soil water around each tree. Xylem water from stem cores close to the ground, therefore, can give a more precise estimate of the isotopic signal of the most recent root water uptake and facilitate more accurate source water attribution.

A Comprehensive Biogeochemical Assessment of Climate‐Threatened Glacial River Headwaters on the Eastern Slopes of the Canadian Rocky Mountains

AbstractClimate change is driving the loss of alpine glaciers globally, yet investigations about the water quality of rivers stemming from them are few. Here we provide an overview assessment of a biogeochemical data set containing 200+ parameters that we collected between 2019 and 2021 from the headwaters of three such rivers (Sunwapta‐Athabasca, North Saskatchewan, and Bow) which originate from the glacierized eastern slopes of the Canadian Rocky Mountains. We used regional hydrometric data sets to accurately model discharge at our 14 sampling sites. We created a Local Meteoric Water Line (LMWL) using riverine water isotope signatures and compared it to collected regional rain, snow, and glacial ice signatures. Principal component analyses of river physicochemical measures revealed distance from glacier explained more data variability than other spatiotemporal factors (i.e., season, year, or river). Discharge, chemical concentrations, and watershed areas were then used to model site‐specific open water season yields for 25 parameters. Chemical yields followed what would generally be expected along river continuums from glacierized to montane altitudinal life zones, with landscape characteristics driving chemical sources and sinks. For instance, particulate chemical yields were generally highest near source glaciers with proglacial lakes acting as settling ponds, whereas most dissolved yields varied by parameter and site. As these headwaters continue to evolve with glacier mass loss, the data set and analyses presented here can be used as a contemporary baseline to mark future change against. Further, following this initial assessment of our data set, we encourage others to mine it for additional biogeochemical studies.

Stable isotope evidence confirms lakes are more sensitive than rivers to extreme drought on the Qinghai-Tibet Plateau, China

The extreme drought event that occurred in China at the beginning of summer 2022 has caused widespread societal concern. The hydrogen and oxygen stable isotope data collected in the summer of 2022 on the eastern Qinghai-Tibet Plateau (QTP) were used here to quantify the responses of the lakes and rivers to extreme drought. The results showed that the evaporation fractionation of lake water was the strongest (lc-excess: −21.62‰), the evaporation fractionation of thermokarst lake water was the second strongest (lc-excess: −15.33‰), and the evaporation fractionation of river water was weak (lc-excess: −0.30‰) during the extreme drought event. Compared to the previous years, the strongest isotopic enrichment of surface water during the extreme drought occurred in lake water (E(δ18O): 58.61%), followed by thermokarst lake water (E(δ18O): 48.18%), while that in river water was weak (E(δ18O): 9.41%). Solar radiation was the main environmental factor controlling isotopic variations among different surface water. The ratios of evaporation to inflow of surface water caused the greatest isotopic enrichment to be found in lakes. Therefore, the occurrence of extreme drought events resulted in the lake water isotopic signal being the strongest evaporation enrichment reaction. The extreme drought event that occurred in the summer of 2022 intensified the evaporation enrichment of different surface waters on the eastern QTP. The results provide new insights into the surface water feedback climate change over the QTP.

Snowpack and groundwater recharge in the Atlas mountains: New evidence and key drivers

Study regionThe Atlas Mountains of Morocco, specifically the High Oum Er-rbiaa (HOER) and Ourika catchments. Study focusto identify the recharge processes within the semi-arid watersheds, in the Atlas Mountains, through monthly monitoring of snow, rainfall, surface water, and groundwater isotope signal, but also the usage of remote sensing data. New hydrological insights for the regionThe spatial-temporal analysis of groundwater and precipitation isotopes reveals significant spatial heterogeneity, primarily influenced by the geological variations in each aquifer. Temporal variations indicate that direct recharge occurs in response to winter precipitation, whereas a delayed response is observed during the summer when snow replenishes groundwater towards the end of the melting season. The findings are further supported by the "Gravity Recovery and Climate Experiment" (GRACE) dataset, which demonstrates that high values of Total Water Storage (TWS) align with groundwater isotopes. This highlights the substantial groundwater abstraction rate between March and June to compensate for the lack of precipitation during this period. The analysis of isotope data indicates that 50% of groundwater recharge in the upstream Jurassic aquifer and 80% in the downstream Triassic-Paleozoic aquifers in the HOER catchment is sourced from snowmelt. Similarly, in the Ourika catchment, snowmelt contributes 30% and 50% of groundwater recharge in the upstream and downstream portions of the catchments respectively. This disparity is due to different melting rates across altitudinal ranges and variations in the lithology of each catchment.

Importance of tree diameter and species for explaining the temporal and spatial variations of xylem water δ18O and δ2H in a multi‐species forest

AbstractIdentifying the vegetation and topographic variables influencing the isotopic variability of xylem water of forest vegetation remains crucial to interpret and predict ecohydrological processes in landscapes. In this study, we used temporally and spatially distributed xylem stable water isotopes measurements from two growing seasons to examine the temporal and spatial variations of xylem stable water isotopes and their relationships with vegetation and topographic variables in a Luxembourgish temperate mixed forest. Species‐specific temporal variations of xylem stable water isotopes were observed during both growing seasons with a higher variability for beeches than oaks. Principal component regressions revealed that tree diameter at breast height explains up to 55% of the spatial variability of xylem stable water isotopes, while tree species explains up to 24% of the variability. Topographic variables had a marginal role in explaining the spatial variability of xylem stable water isotopes (up to 6% for elevation). During the drier growing season (2020), we detected a higher influence of vegetation variables on xylem stable water isotopes and a lower temporal variability of the xylem water isotopic signatures than during the wetter growing season (2019). Our results reveal the dominant influence of vegetation on xylem stable water isotopes across a forested area and suggest that their spatial patterns arise mainly from size‐ and species‐specific as well as water availability‐dependent water use strategies rather than from topographic heterogeneity. The identification of the key role of vegetation on xylem stable water isotopes has critical implications for the representativity of isotopes‐based ecohydrological and catchments studies.

A Snapshot on the Buildup of the Stable Water Isotopic Signal in the Upper Snowpack at EastGRIP on the Greenland Ice Sheet

AbstractThe stable water isotopic composition in firn and ice cores provides valuable information on past climatic conditions. Because of uneven accumulation and post‐depositional modifications on local spatial scales up to hundreds of meters, time series derived from adjacent cores differ significantly and do not directly reflect the temporal evolution of the precipitated snow isotopic signal. Hence, a characterization of how the isotopic profile in the snow develops is needed to reliably interpret the isotopic variability in firn and ice cores. By combining digital elevation models of the snow surface and repeated high‐resolution snow sampling for stable water isotope measurements of a transect at the East Greenland Ice‐core Project campsite on the Greenland Ice Sheet, we are able to visualize the buildup and post‐depositional changes of the upper snowpack across one summer season. To this end, 30 cm deep snow profiles were sampled on six dates at 20 adjacent locations along a 40 m transect. Near‐daily photogrammetry provided snow height information for the same transect. Our data shows that erosion and redeposition of the original snowfall lead to a complex stratification in the δ18O signature. Post‐depositional processes through vapor‐snow exchange affect the near surface snow with d‐excess showing a decrease in surface and near‐surface layers. Our data suggests that the interplay of stratigraphic noise, accumulation intermittency, and local post‐depositional processes form the proxy signal in the upper snowpack.

Flushing or mixing? Stable water isotopes reveal differences in arctic forest and peatland soil water seasonality

AbstractUnderstanding the relative importance of different water sources that replenish soil water storage is necessary to assess the vulnerability of sub‐arctic areas to changes in climate and altered rain and snow conditions, reflected in the timing and magnitude of water infiltration. We examine spatiotemporal variability and seasonal origin of soil water at the soil‐vegetation interface in Pallas catchment, located in northern Finland. The field study was conducted from May 2019 to June 2020 over two snowmelt seasons and one summer growing season. We sampled soil cores up to a 1‐m depth and stem water of dominant tree species at four sites located in forests and forested peatlands for stable water isotopes. Seasonal rainfall variation and late snowmelt events were well identifiable in the well‐drained soils of forested areas, while this input signal is heavily attenuated in wetter, forested peatland areas. Spatiotemporal variability of soil water in two forest sites was similar, whereas soil water storage in peatland sites was controlled by the extent of hydrologic connectivity to the adjoining water pools. A mixture of both summer and winter precipitation was present in peatlands during the entire study period, while forest mineral soils showed an ephemeral response to water input and got nearly fully flushed twice during the hydrological year. Meltwater signal in forest soils was dominant after snowmelt and in early spring but became displaced by isotopically enriched rainfall during the summer. Seasonal evolution of soil water pools was not reflected in tree stem dynamics but offset between soil and stem water isotopic signals was less pronounced in forested peatlands. This field data set uncovered seasonal changes of soil water isotopic signal at high depth‐resolution, quantifying the importance of snowmelt water in replenishing and sustaining soil water storage in sub‐arctic conditions.

Towards hydrological connectivity in the karst hillslope critical zone: Insight from using water isotope signals

Hydrologic connectivity along the hillslope critical zone is essential for the transmission of water, solutes, and nutrients to streams. However, the influences of critical zone structure on hydrological connectivity are not adequately understood, especially for hillslopes with complex topography. We investigated the hydrological connectivity along different topographical positions of two adjacent dolomite hillslope plots in southwest China: a deep soil plot (average value: 66 cm) and shallow soil plot (average value: 35 cm). We characterized the dynamics of multiple-interface runoff and soil water based on observations of stable isotopes (δ18O and δD) and hydrometric monitoring, and computed the lc-excess, young water fraction (yw), and hydrograph separation from June 2020 to July 2021. On both hillslopes, soil depth gradually increased from up-slope to down-slope, and the contribution of event water and yw to soil water gradually decreased (100 % to 69 %). In the vertical depth, a sudden decrease in the contribution of yw (98 % to 47 %) and event water (44 % to 11 %) in the down-slope of the soil-epikarst interface was observed. These results indicate that the infiltration of rainwater from up-slope to down-slope was altered by vertical flow to lateral flow, which was connected through the soil-epikarst interface and finally converged in the low-lying area. Contrary to expectations, the contribution of yw at the soil-epikarst interface was lower in the shallow soil plot (43 %) than in the deep soil plot (71 %). As a supplement, moisture at the soil-epikarst interface was significantly higher in the shallow soil plot. This indicates that the renewal cycle of groundwater was slower in shallow soil-covered hillslopes not only because almost all rainwater (∼98 %) entered the soil-epikarst to redistribute as well as the higher water storage area in the soil-epikarst interface. Accordingly, the soil-epikarst structure, which is differentiated by soil depth, controls hillslope hydrological connectivity and groundwater storage capacity in the karst region of southwest China.

The role of radiative cooling and leaf wetting in air–leaf water exchange during dew and radiation fog events in a temperate grassland

During prolonged dry periods, non-rainfall water (NRW) plays a vital role as water input into temperate grasslands, affecting the leaf surface water balance and plant water status. Previous chamber and laboratory experiments investigated air–leaf water exchange during dew deposition, but overlooked the importance of radiative cooling on air–leaf water exchange because the chamber is a heat trap, preventing radiative cooling. To complement these previous studies, we conducted a field study, in which we investigated the effect of radiatively-induced NRW inputs on leaf water isotope signals and air–leaf water exchange in a temperate grassland during the dry-hot summers of 2018 and 2019. We carried out field measurements of the isotope composition of atmospheric water vapor, NRW droplets on foliage, leaf water, xylem water of root crown, and soil water, combined with meteorological and plant physiological measurements. We combined radiation measurements with thermal imaging to estimate leaf temperatures using different methods, and computed the corresponding leaf conductance and air–leaf water exchange. Our results indicate that radiative cooling and leaf wetting induced a switch of direction in the net water vapor exchange from leaf-to-air to air-to-leaf. The leaf conductance and air–leaf water exchange varied by species due to the species-specific biophysical controls. Our results highlight the ecological relevance of radiative cooling and leaf wetting in natural temperate grasslands, a process which is expected to influence land surface water budgets and may impact plant survival in many regions in a drier climate.

Fingerprints of Frontal Passages and Post‐Depositional Effects in the Stable Water Isotope Signal of Seasonal Alpine Snow

AbstractStable water isotopes are used as a paleothermometer in ice cores for climate reconstructions over the past millennia. The underlying physical processes involved in the isotope‐temperature relation, however, unfold at much shorter timescales. Here, we study the temporary archival of frontal passages in the seasonal Alpine snow cover. We combine five snow profiles sampled in winter 2017 at the Weissfluhjoch with a quantitative snow layer age reconstruction and atmospheric reanalysis data to characterize the circulation and clouds associated with the precipitation producing synoptic‐scale cold and warm fronts. We find that the vertical cloud structure and the air parcels' net cooling during transport leave a distinct imprint in the δ18O and δD vertical profile in the snow. The near‐surface humidity gradient at the moisture source is reflected in the second order isotope parameter deuterium excess. In the cold season, these environmental conditions during cloud formation and at the moisture source are preserved in the snow. In the melt season, significant post‐depositional effects due to wet snow metamorphism, however, leads to an enrichment in heavy isotopes in the snow and a strong smoothing of the initial atmospheric imprint. These findings show that the isotope signal archived in the dry snow cover is strongly modulated by individual weather systems prior to deposition. Major shifts in the upper‐level jet stream and cyclone tracks likely leading to changes in moisture source regions and conditions, could therefore be detectable in the isotope composition of Alpine ice.

Disentangling the impact of air–sea interaction and boundary layer cloud formation on stable water isotope signals in the warm sector of a Southern Ocean cyclone

Abstract. Stable water isotopes in marine boundary layer water vapour are strongly influenced by the strength of air–sea fluxes. Air–sea fluxes in the extratropics are modulated by the large-scale atmospheric flow, for instance by the advection of warm and moist air masses in the warm sector of extratropical cyclones. A distinct isotopic composition of the water vapour in the latter environment has been observed over the Southern Ocean during the 2016/2017 Antarctic Circumnavigation Expedition (ACE). Most prominently, the secondary isotope variable deuterium excess (d=δ2H–8⋅δ18O) shows negative values in the cyclones’ warm sector. In this study, three mechanisms are proposed and evaluated to explain these observed negative d values. We present three single-process air parcel models, which simulate the evolution of δ2H, δ18O, d and specific humidity in an air parcel induced by decreasing ocean evaporation, dew deposition and upstream cloud formation. Simulations with the isotope-enabled numerical weather prediction model COSMOiso, which have previously been validated using observations from the ACE campaign, are used to (i) validate the air parcel models, (ii) quantify the relevance of the three processes for stable water isotopes in the warm sector of the investigated extratropical cyclone and (iii) study the extent of non-linear interactions between the different processes. This analysis shows that we are able to simulate the evolution of d during the air parcel's transport in a realistic way with the mechanistic approach of using single-process air parcel models. Most importantly, we find that decreasing ocean evaporation and dew deposition lead to the strongest d decrease in near-surface water vapour in the warm sector and that upstream cloud formation plays a minor role. By analysing COSMOiso backward trajectories we show that the persistent low d values observed in the warm sector of extratropical cyclones are not a result of material conservation of low d. Instead, the latter Eulerian feature is sustained by the continuous production of low d values due to air–sea interactions in new air parcels entering the warm sector. These results improve our understanding of the relative importance of air–sea interaction and boundary layer cloud formation on the stable water isotope variability of near-surface marine boundary layer water vapour. To elucidate the role of hydrometeor–vapour interactions for the stable water isotope variability in the upper parts of the marine boundary layer, future studies should focus on high-resolution vertical isotope profiles.

Hydrochemical Characteristics and the Genetic Mechanism of Low–Medium Temperature Geothermal Water in the Northwestern Songliao Basin

The geothermal resources in sedimentary basins have high potential for development and utilization, and have become an important research topic globally. This study focuses on the geothermal system in the northwestern Songliao Basin. Water chemistry and isotopic signatures of geothermal fluids and shallow groundwater are analyzed. Water–rock interactions, recharge sources, and the ages of geothermal fluids are revealed and recharge elevation, circulation depth, and the reservoir temperature of the geothermal fluids are estimated. This article proposes deep heat sources and genetic mechanism for geothermal system. The results are as follows: The hydrochemical types of geothermal water mainly included Cl·HCO3-Na, HCO3·Cl-Na, and Cl-Na, and the TDS gradually increased from the margin to the center of the basin and from anticlines to the depression on both sides. The geothermal water was recharged by paleo-atmospheric precipitation in the northwest mountainous area at an elevation of 300–700 m. The 14C ages showed that the geothermal water flowed at an extremely low rate (millennial scale) and had a low circulation rate. The temperature of the geothermal reservoirs was estimated to be 45.19–83 °C using a quartz geothermometer. The geothermal water had a genetic model of stratum-controlling geothermal reservoirs, lateral runoff recharge, and heat supply by terrestrial heat flow. The underlying reasons for the high geothermal gradient and terrestrial heat flow in the basin include the uplift of the Moho, the uplift of the upper mantle, and the presence of a high-electrical-conductivity layer in the crust.

Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

Abstract. Tropical ice clouds have an important influence on the Earth's radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales. These processes are not yet fully understood and lead to large uncertainties in climate projections. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform convection-permitting simulations with the regional Consortium for Small-Scale Modelling isotope-enabled (COSMOiso) model for the period from June to July 2016. First, we evaluate our model simulations using space-borne observations of mid-tropospheric water vapour isotopes, monthly station data of precipitation isotopes, and satellite-based precipitation estimates. Next, we explore the isotope signatures of tropical deep convection in atmospheric water vapour and ice based on a case study of a mesoscale convective system (MCS) and a statistical analysis of a 1-month period. The following five key processes related to tropical ice clouds can be distinguished based on isotope information: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdraughts that enriches the environmental vapour, and (5) the freezing of liquid water just above the 0 ∘C isotherm in convective updraughts. Importantly, we note large variations in the isotopic composition of water vapour in the upper troposphere and lower tropical tropopause layer, ranging from below −800 ‰ to over −400 ‰, which are strongly related to vertical motion and the moist processes that take place in convective updraughts and downdraughts. In convective updraughts, the vapour is depleted by the preferential condensation and deposition of heavy isotopes, whereas the non-fractionating sublimation of ice in convective downdraughts enriches the environmental vapour. An opposite vapour isotope signature emerges in thin-cirrus cloud regions where the direct transport of enriched (depleted) vapour prevails in large-scale ascent (descent). Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the West African monsoon water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.