Isotopic Composition Of Water Research Articles

Absence of a strong, deep-reaching Antarctic Circumpolar Current zonal flow across the Tasmanian gateway during the Oligocene to early Miocene

The vigorous eastward flow of the Antarctic Circumpolar Current (ACC) connects all major ocean basins and plays a prominent role in the transport of heat, carbon and nutrients around the globe. However, the establishment of a deep circumpolar flow, similar to the present-day ACC, remains controversial thereby obscuring our understanding of its climatic impact. Deciphering the chemical composition of Circumpolar Deep Water (CDW) within the ACC can provide critical insights about its development and evolution. Here we present new fossil fish teeth/bone debris neodymium isotope (εNd) records from Deep Sea Drilling Project (DSDP) Sites 278 and 274 in the southwest Pacific Ocean, with the aim to trace changes in deep water masses across the Tasmanian Gateway between the early Oligocene and early Miocene (~ 33–22 Ma). Site 274 provides the first Nd isotope record proximal to the Ross Sea during the Oligocene (33.5–23.4 Ma). Its Nd isotope composition shows excursions to very radiogenic values, εNd(t) = −3.1 and εNd(t)= − 3.7, at 33.5 Ma and 23.8 Ma, respectively, in response to major steps in Antarctic ice sheet expansion. A shift to lower, more unradiogenic εNd(t) values between 29.7 and 29.1 Ma is linked to an increased influence of proto-CDW upwelling at the site. In contrast, the Nd isotope record from Site 278 in the southern Emerald Basin shows little variability (εNd(t) = −6.0 to −6.7) throughout the Oligocene and early Miocene (30.9–21.8 Ma). Comparison with published data north of the ACC path, demonstrates the presence of two deep water masses in the South Pacific prior to the inferred onset of the ACC (33–30 Ma), one occupying depths between ~2500 and 3000 m (εNd(t)= ~ −3 to −5) and a deep/bottom water mass (> 3000 m) with a more unradiogenic Nd isotope composition (εNd(t)= ~ −6). Site 278 located close to the proto-polar front (proto-PF) indicates that following the inferred onset of the ACC, deep waters bathing the southern Emerald Basin remained more radiogenic in the Southwest Pacific compared to sites along the proto-PF in the South Atlantic and Indian Ocean (εNd(t)= ~ −8.1). This indicates a provinciality in Nd isotope compositions of deep waters along the proto-PF across the Tasmanian Gateway. Our data are incompatible with the existence of a modern-like homogenous (lateral and vertical) Nd isotope composition of CDW along the main flow path of the ACC in all oceanic basins in the Oligocene to early Miocene. We attribute distinct Nd isotope compositions of deep waters across the Tasmanian Gateway to reflect a less deep reaching and weaker ACC (proto-ACC) than today. Our findings suggest that the modern strong and deep-reaching ACC flow must have been developed at a later point in the Neogene.

黄土丘陵区优势造林树种水分来源对季节性干旱的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 黄土丘陵区优势造林树种水分来源对季节性干旱的响应 DOI: 10.5846/stxb202106221657 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 山西省高等学校科技创新（2020L0141，2020L0146）；山西农业大学科技创新基金（博士科研启动）（2020BQ44）；山西省优秀博士来晋工作奖励（SXYBKY2020007） Responses of water source to seasonal drought of dominant afforestation tree species in the loess hilly region of China Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:黄土丘陵区处于季节性干旱生态脆弱地带，探明水分环境变化对区域优势造林树种水分来源的影响，对干旱区植物水分利用及其共生关系具有重要意义。以该地区广泛种植的沙棘（Hippophae rhamnoides）+油松（Pinus tabuliformis）、沙棘（H.rhamnoides）+刺槐（Robinia pseudoacacia）混交林（HrPt，HrRp）及沙棘（Hr）、油松（Pt）和刺槐（Rp）纯林为对象，测定其茎秆水、土壤水和降水稳定同位素组成，采用IsoSource模型量化水分来源，相似性比例指数（PS）定量分析共生植物间水分利用关系。结果表明：（1）旱季，同一树种纯林和混交林间吸水层位存在差异，纯林中主要利用0-100 cm土层土壤水，其中Hr和Rp对40-100 cm的利用率为46.8%和43.8%，Pt对0-40 cm为83.9%；混交林中则更倾向于利用40-200 cm，其中HrPt（沙棘）对100-200 cm为58.8%，HrPt（油松）对40-100 cm为47.4%；HrRp（沙棘）对100-200 cm为58.2%，HrRp（刺槐）对100-200 cm为65.3%。（2）湿润季，纯林和混交林中各树种主要利用0-100 cm土层土壤水，利用率均在70%以上。（3）旱季PS指数显著大于湿润季，且HrRp的PS指数最大，表明沙棘和刺槐之间存在水源竞争现象。综上，相比HrRp，HrPt中共生植物间主要通过水分利用空间上的差异应对季节性干旱，使其组成的生态系统更好地适应区域脆弱的生态环境。研究结果可为区域退化生态恢复与重建及干旱适应性措施体系构建等提供依据。 Abstract:The loess hilly region is located in an ecologically fragile area with seasonal drought. Clarifying the impact of changes in the water environment on the water source of regional dominant afforestation tree species is of great significance to the water use of plants and their symbiosis in arid areas. Therefore, two kinds of mixed forests (Hippophae rhamnoides + Pinus tabuliformis, HtPt, H. rhamnoides+Robinia pseudoacacia, HtRp), the pure forests of H. rhamnoides (Ht), P. tabuliformis (Pt), and R. pseudoacacia (Rp) were selected. The isotope composition of xylem water, soil water and precipitation were determined. The water use and the relationship among symbiotic plants were quantitatively analyzed by using the IsoSource model and the proportional similarity (PS) index. The results indicated that:(1) during the dry season, there were differences in water absorption layer between pure forest and mixed forest of the same tree species. The tree species in the pure forest mainly used soil water in the 0-100 cm. The utilization ratios of Hr and Rp to 40-100 cm were 46.8% and 43.8%, and Pt from 0-40 cm was 83.9%. While, in the mixed forests were more inclined to use 40-200 cm. Among them, the utilization rate of HrPt (H. rhamnoides) from 100-200 cm layer was 58.8%, HrPt (P. tabuliformis) from 40-100 cm was 47.4%. HrRp (H. rhamnoides) and HrRp (R. pseudoacacia) from 100-200 cm were 58.2% and 65.3%, respectively. (2) During the wet season, tree species in pure and mixed forests were mainly use soil water in the 0-100 cm, and the utilization rates was above 70%. (3) The PS index in the dry season was significantly greater than that in the wet season, and in the HrRp was the highest, indicating that there might be water competition between H. rhamnoides and R. pseudoacacia. In summary, compared with HrRp, symbiotic plants in HrPt mainly respond to seasonal droughts through spatial differences in water use, which made it much easier for this ecosystem to be adaptive to the fragile ecological environment in the region. The research results can provide a basis for the restoration and reconstruction of regional degraded ecology and the construction of drought-adaptive measures system. 参考文献 相似文献 引证文献

Fe3+/FeT ratios of amphiboles determined by high spatial resolution single-crystal synchrotron Mössbauer spectroscopy

Abstract The Fe3+/FeT ratios (Fe3+/[Fe2++Fe3+]) in minerals can be used to understand their crystallization and post-crystallization conditions. However, as natural minerals are often zoned and contain inclusions, bulk techniques, e.g., wet chemistry, may not provide accurate Fe3+/FeT values for a single phase of interest. We determined Fe3+/FeT ratios of amphiboles in different crystallographic orientations by single-crystal synchrotron Mössbauer spectroscopy (SMS) in energy and time domain modes from four volcanic localities (Long Valley Caldera, Mount St. Helens, Lassen Volcanic Center, U.S.A., and Mt. Pinatubo, Philippines). The high spatial resolution (as low as 12 × 12 μm spot size) and standard-free nature of SMS allow the detection of intra-grain compositional heterogeneities in Fe3+/FeT with relatively low uncertainties. We combine SMS with major element compositions, water contents, and hydrogen isotope compositions to document the Fe3+/FeT ratios as a function of mineral composition and post-crystallization dehydrogenation. Spectra were fitted with up to five distinct sites: ferrous iron on M(1), M(2), M(3), and ferric iron on M(2) and M(3), consistent with X-ray diffraction studies on single crystals of amphibole. The Fe3+/FeT ratios range from 0.14 ± 0.03 (Long Valley Caldera), 0.51 to 0.63 ± 0.02 (representing intra-grain heterogeneities, Mount St. Helens) to 0.86 ± 0.03 (Lassen Volcanic Center). The latter grain experienced post-crystallization dehydrogenation, shown by its low water content (0.6 ± 0.05 wt%) and its elevated hydrogen isotope composition (δD = +25 ± 3‰ relative to SMOW). The Fe3+/FeT ratios of 0.62 ± 0.01 and 0.20 ± 0.01 of two Mt. Pinatubo grains correlate with high-Al2O3 cores and low-Al2O3 rims and smaller phenocrysts in the sample, respectively. This study shows that SMS is capable of distinguishing two different domains with dissimilar Fe3+/FeT values formed under different crystallization conditions, demonstrating that SMS in combination with major element, water, and hydrogen isotope compositions allows the interpretation of amphibole Fe3+/FeT ratios in the context of crystallization and post-crystallization processes.

Use of water isotopes and chemistry to infer the type and degree of exchange between groundwater and lakes in an esker complex of northeastern Ontario, Canada

Abstract. While interactions between groundwater and lake-water influence water chemistry, water balance, aquatic organisms, biochemical cycles and contamination levels, they remain a poorly studied component of lake hydrology. Identifying the controls of groundwater and lake-water interactions at the landscape level and classifying lakes into categories based on their degree of interaction with the groundwater can provide insights into a lake's sensitivity and vulnerability to environmental stressors. Such information can also provide baseline conditions for comparison to future changes that are important for water management and conservation. To this end, water chemistry and water isotopic composition were investigated in a set of 50 boreal lakes located at different elevations in an esker system near Timmins, Ontario. Analyses focused on stable isotopic ratios of hydrogen and oxygen and specific conductance as indicators of the position of a lake with respect to the influence of groundwater. Both isotopic composition and specific conductance distinguished higher-elevation groundwater-recharge lakes from lower-elevation groundwater-discharge lakes. Groundwater-recharge lakes were high-elevation lakes characterized by enriched isotopic values and low values of specific conductance. In contrast, groundwater-discharge lakes were isotopically depleted and had higher values of specific conductance and occurred at lower elevations. An intermediate group of lakes was also defined (termed seepage lakes) and had intermediate isotopic and water-chemistry characteristics compared to recharge and discharge lakes. Differences in water geochemistry between field campaigns revealed that upland groundwater-recharge lakes showed evidence of evaporative drawdown, indicating sensitivity to short-term changes in climate, whereas the lower-elevation groundwater-discharge lakes showed little variation between seasonal samples and consequently would likely be affected only by hydroclimatological changes of greater duration and magnitude.

Estimates of the contribution of fog to wet atmospheric deposition in Czech mountain forests based on its stable hydrogen and oxygen isotope composition: preliminary results

Stable isotopes are increasingly being used in many scientific fields, including environmental sciences. In this study we measured the variation in the stable hydrogen and oxygen isotope composition of fog water, rain water (in the form of bulk falling precipitation) and throughfall water in the Šumava (the Bohemian Forest), Krkonoše (the Giant Mts.) and Jizerske hory (the Jizera Mts.) Mts. in October-November 2017. In total, 46 cumulative two-week samples were collected and analysed. Our results indicate that the overall stable hydrogen and oxygen isotope composition of fog and rain samples differed significantly, fog being isotopically enriched in the heavier isotopes 2H and 18O relative to rain. In contrast to our assumption, throughfall water was generally depleted in the heavier isotopes 2H and 18O relative to rainwater. Hence, the simple mixing model for most samples yielded an unrealistic percentage outside the reasonable range of 0–100%. For few samples, however, the estimated contribution of fog to throughfall ranged between 3 and 8% based on δ2H and 4 and 7% based on δ 18O, which is lower than that estimated for the same mountain regions by other authors using different methods and significantly lower than that reported for mountain ranges in neighbouring countries. Although using stable isotopes is a promising tool for determining the contribution of fog to the hydrological budget when assessing atmospheric deposition, the critical limitations are in the collection, manipulation and storing of the samples.

Hydrological Separation of Event and Pre-event Soil Water in a Cultivated Landscape

Soil water mixing is a complex process that is dependent on vegetation, soil physiochemical characteristics, and climate. In this research, we utilize natural tracers, δ18O and δ2H, to determine if hydrological separation of recent precipitation event and pre-event water occurred within the soil profile of a maize field in central Ohio. Soil samples were collected at three sites and four depths, and the soil water was extracted via cryogenic distillation. Soil water isotopic composition showed mixing of precipitation event and pre-event water that reflected weighted average precipitation isotopic composition. The range in isotopic signatures also indicated that hydrological mixing within the soil profile was facilitated by soil physical characteristics and effective gravitational drainage of event water. These results support previous work that hydrological separation does not occur in soil water in temperate climates and contribute to ongoing research to conceptualize soil water cycling in monoculture systems.

New Data on Hydrogeochemical and Isotopic Composition of Natural Waters of the Baidar Valley (Crimean Peninsula)

Abstract —Results of study of natural waters of the Baidar valley (southwestern Crimean Peninsula) obtained during the 2018–2019 field works are presented. Major groundwater resources of the study area are confined to the Upper Jurassic aquifer complex, which serves as a recharge source for the aquifer systems of the Plain Crimean and the Azov–Kuban’ artesian basins and hydrogeologic folded region of the Crimean Mountains mega-anticlinorium. The regional waters are fresh and ultrafresh, predominantly of calcium bicarbonate composition, with TDS varying from 208 to 1269 mg/dm3. The study enabled their classification into eight geochemical groups: (1) waters of a regional fracture zone in carbonate-terrigenous rocks affected by continental salinization; (2) waters of a regional fracture zone affected by leaching of aluminosilicates and sulfide oxidation; (3) waters of a regional fracture zone dominated by sodium aluminosilicates in the fracture filling (long-term interaction in the water–rock system), affected by continental salinization; (4) regional fracture zones dominated by sodium aluminosilicates affected by anthropogenic continental salinization; (5) groundwaters in fracture–vein aquifers affected by leaching of aluminosilicates and sulfide oxidation; (6) fracture–vein aquifers affected by leaching of sodium aluminosilicates (long-term interaction in the water–rock system); (7) waters in fractured karst aquifers; and (8) surface waters subjected to continental salinization. Fracture karst waters, which were found to be most protected against human impact and continental salinization processes, are slightly alkaline (pH = 7.7), fresh (with average TDS = 444 mg/dm3), with low silicon concentrations (2.23 mg/dm3), and of calcium bicarbonate composition. Waters residing in regional fracture and fracture–vein zones are affected by continental salinization and anthropogenic load and are neutral to alkaline (pH = 7.1–8.6), predominantly fresh (TDS = 269–1269 mg/dm3), with average silicon concentrations of 4.61–4.70 mg/dm3, of calcium bicarbonate composition, with high concentrations of sulfate ion, magnesium, and sodium. The waters of the Chernaya River, Chernorechensk reservoir, and lakes, which are also affected by continental salinization, are slightly alkaline (pH = 8.3), brackish (TDS = 207–364 mg/dm3), with an average silicon concentration of 1.18 mg/dm3, of calcium bicarbonate composition, with high concentrations of chlorine ion, magnesium, and sodium. The calculated intensity of chemical-element migration in waters of the background composition follows the descending order: very strong, I17.7 > Br14.4; strong, Se2.83 > B2.22 > Sr1.46 > Sb1.12 > Be1.07 > Hg1.06; moderately strong, Zn0.74 > Mo0.50 > Li0.46 > Sc0.41 > Ag0.18 > As0.16 > Si0.123 > Ba0.122; weak, Cr0.10 > Cu0.096 > Bi0.080 > Sn0.068 > Tl0.067 > P0.062 > Ni0.043 > Ta0.040 > Ge0.034 > Cd0.028 > Fe0.026 > Rb0.024 > Co0.023 > Pb0.020 > W0.017 > V0.012; very weak (inert), Nb0.008 > Hf0.0033 > Mn0.0031 > La0.0029 > Cs0.0022 > Ti0.0018 > Ga0.0016 > Y0.0013 > Al0.0008 > Zr0.0008. All the studied waters are found to be of atmospheric origin and located along the global (GMWL) and local (LMWL) meteoric water lines. Their δ18O value varies from –9.9 to –3.3‰, and δD value, from –64.2 to –32.5‰. Sedimentary carbonate rocks, atmospheric carbon dioxide, organic compounds, and hydrolysis of aluminosilicate minerals serve as the source of δ13C bicarbonate ion in natural waters of the Baidar valley. Surface waters have a heavier carbon isotope composition (δ13C = –9.2 to –6.2‰), which is due to atmospheric CO2, plant growth, and associated microbial activity. Fracture karst waters are characterized by a lighter carbon isotope composition (δ13C = –12.8 to –11.0‰) because of their interaction with dispersed organic matter. Waters of the regional fracture and fracture–vein zones display the widest variation in δ13C (–15.5 to –6.9‰), which is associated with a mixed type of “isotope supply” to the waters. A complex hydrogeochemical field that has formed in the Baidar valley tends to be increasingly affected by the anthropogenic factor.

Search for hydrogeochemical indicators of the genetic relation between mud volcanism and oil and gas fields

The paper reports the results of a comparative analysis of the chemical and isotope composition (δ180 and δD) of mud volcanic waters and formation waters from oil and gas fields. Studies show that the waters discharged by mud volcanoes in most cases are very similar to formation waters. The most characteristic geochemical traits of both waters are elevated concentrations of hydrocarbonate ions, iodine, boron, bromine, and a low content of sulfate ions.

Solar wind contributions to Earth’s oceans

The isotopic composition of water in Earth’s oceans is challenging to recreate using a plausible mixture of known extraterrestrial sources such as asteroids—an additional isotopically light reservoir is required. The Sun’s solar wind could provide an answer to balance Earth’s water budget. We used atom probe tomography to directly observe an average ~1 mol% enrichment in water and hydroxyls in the solar-wind-irradiated rim of an olivine grain from the S-type asteroid Itokawa. We also experimentally confirm that H+ irradiation of silicate mineral surfaces produces water molecules. These results suggest that the Itokawa regolith could contain ~20 l m−3 of solar-wind-derived water and that such water reservoirs are probably ubiquitous on airless worlds throughout our Galaxy. The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios. Water and hydroxyl enrichment in the solar-wind-irradiated rim of an olivine grain from asteroid Itokawa suggests that its regolith could contain ~20 l m−3 of water from solar wind—a potential water source for airless planetary bodies.

Effects of phenotypic variability on the oxygen and hydrogen isotope compositions of grains in different winter wheat varieties

ABSTRACT Stable isotope analyses are the leading method for geographic origin determination, especially of plant-based agricultural products. Origin analysis is typically done by comparing a suspicious sample to reference materials with known geographic origin. Reference materials are usually collected at the species level, assuming different varieties of a species to have comparable isotope compositions within a given location. We evaluated whether different phenotypes that are expressed in different varieties of winter wheat (Triticum aestivum L.) influence the oxygen (δ 18O) and hydrogen (δ 2H) isotope composition of plant tissue water and organic compounds. We found that mean δ 18O and δ 2H values among winter wheat varieties did not differ significantly in leaf water, however, differed significantly in bulk dried grain tissue. The differences in bulk dried grain δ 18O and δ 2H values among varieties can be related to differences in phenotypic trait expression among varieties. Despite this substantial phenotypic variability, the overall variability of bulk dried grain δ 18O and δ 2H values among varieties was small (SD 0.54 ‰ for oxygen, 3.60 ‰ for hydrogen). We thus conclude that reference materials collected at the species level should be sufficient for geographic origin analysis of winter wheat and possibly other cereals using δ 18O and δ 2H values.

No evidence of isotopic fractionation in olive trees (Olea europaea): a stable isotope tracing experiment

ABSTRACT Plant transpiration is the dominant water flux in the global terrestrial water balance and a key process in the hydrological sciences. Stable isotopes have contributed greatly to this understanding but one difficult assumption for plant water source quantification using hydrogen and oxygen isotopes is that no isotopic fractionation occurs during water uptake and transport within the plant. Here we present a simple glasshouse experiment with two potted olive trees to test isotopic fractionation. We irrigated the trees with labelled water and cryogenically extracted water from twigs, cores and roots. We found no significant differences in the isotopic composition of water extracted from wood cores and twigs in distinct parts of the trees as they reflected the signature of labelled water. However, significant differences were obtained between plant water and deep soil water. Our results suggest no isotopic fractionation in olive trees, under the specific experimental conditions, validating the traditional isotope-tracing approach.

Isotopic evidence of increasing water abundance and lake hydrological change in Old Crow Flats, Yukon, Canada

Lake-rich northern permafrost landscapes are sensitive to changing climate conditions, but ability to track real-time and potentially multiple hydrological responses (e.g. lake expansion, drawdown, drainage) is challenging due to absence of long-term, sustainable monitoring programs in these remote locations. Old Crow Flats (OCF), Yukon, is a Ramsar Wetland of International Importance where concerns about low water levels and their consequences for wildlife habitat and traditional ways of life prompted multidisciplinary studies during the International Polar Year (2007–2008) and led to the establishment of an aquatic ecosystem monitoring program. Here, we report water isotope data from 14 representative thermokarst lakes in OCF, the foundation of the monitoring program, and time-series of derived metrics including the isotope composition of input waters and evaporation-to-inflow ratios for a 13 year period (2007–2019). Although the lakes spanned multiple hydrological categories (i.e. rainfall-, snowmelt- and evaporation-dominated) based on initial surveys, well-defined trends from application of generalized additive models and meteorological records reveal that lakes have become increasingly influenced by rainfall, and potentially waters from thawing permafrost. These sources of input have led to more positive lake water balances. Given the documented role of rainfall in causing thermokarst lake drainage events in OCF and elsewhere, we anticipate increased vulnerability of lateral water export from OCF. This study demonstrates the value of long-term isotope-based monitoring programs for identifying hydrological consequences of climate change in lake-rich permafrost landscapes.

Spatial distribution of stable isotopes in surface water on the upper Indus River basin (UIRB): Implications for moisture source and paleoelevation reconstruction

Appropriate characterisation of isotope signatures (δ18O and δD) of groundwater, glaciers, and surface water in the Himalayan catchment are important for refining the sources of moisture and developing a modern isotope–elevation relationship for paleoelevation reconstructions. The Indus River located in Western Himalaya is previously unexplored in respect of the variation in water isotopic composition with elevation. In this study, water samples (n = 96) were collected from mainstream, tributaries, streams, glaciers, groundwater, lakes, hot springs, and pond water during the summer season. Results showed that δ18O signatures of the mainstream, tributaries and groundwater range from −15.1‰ to −12.3‰, −16.5‰ to −12‰, −13.8‰ to −12‰, with a mean value of −13.6‰, −14.7‰, −12.8‰, respectively. There was a northward rise in δ18O by 1.30‰ per degree of latitude in the tributary waters. The d-excess and δ18O values suggested that the key source of moisture in the studied river basin was mainly from westerlies and southwest monsoons. Thus, regional moisture recycling and subcloud evaporation contributed little in supplying moisture to the precipitation. The isotopic signature (δ18O) of the tributary stream and its surrounding regions conformed to a best-fit second-order polynomial relationship between δ18O and elevation over 3500 masl. Moreover, an δ18O elevation slope of −2‰/km is also established in this study area. This isotope–elevation relationship will be highly useful and can be applied to the Indus River and other Himalayan rivers for paleoelevation reconstruction.

A mechanistic understanding of oxygen isotopic changes in the Western United States at the Last Glacial Maximum

At the Last Glacial Maximum (LGM), records suggest drier conditions in the northwest United States and wetter conditions in the southwest United States relative to present-day as well as widespread changes in the isotopic composition of water. However, the mechanisms responsible for these changes remain ambiguous. Here, we explore differences in western United States hydroclimate between the LGM and preindustrial with a water isotope tracer enabled Earth System Model. We then use proxy forward models to compare simulated and recorded δ18O in speleothems. We find that the pattern of hydroclimate response in the western United States at the LGM relates to a combination of 1) increased frequency and southward shifted wintertime extratropical cyclones in the North Pacific, 2) greater rainout of moisture as it moves over the continent, and 3) reduced evaporation in the cooler LGM climate. The simulated lower δ18O of precipitation at the LGM relates predominantly to an increase in cool season moisture removal efficiency, with a secondary contribution from relatively more cool season precipitation. Both surface temperatures and North American ice sheets contribute to these hydroclimate changes at the LGM. Comparisons between δ18O from proxy forward models and speleothem records in the western United States show general agreement at the LGM, with increasing depletion moving towards the continental interior. This study highlights the similarities and differences between hydrologic and δ18O changes at the LGM and emphasizes the utility of model-proxy comparison for interpretation.

Hydrothermal and magmatic contributions to surface waters in the Aso caldera, southern Japan: Implications for weathering processes in volcanic areas

The role of hydrothermal fluids in the dissolution of primary minerals in volcanic areas, hotspots of silicate weathering at the global scale, is not well quantified. This study aims to determine the hydrothermal contribution to the lateral export of solutes from the Aso caldera, southern Japan. A set of indicators for the hydrothermally related contribution is systematically discussed. Spring, groundwater and river water samples were collected along the two main rivers of the caldera, and their major and trace element concentrations, as well as the stable isotopic compositions of water (δ2Η and δ18O), dissolved inorganic carbon (δ13C), and sulphate (δ34S and δ18O) were analysed. Hydrothermal springs in the study area are associated with a substantial sulphate contribution of magmatic origin. In contrast, the hydrochemistry of non-hydrothermal springs reflects surface water-soil interactions. In particular, non-hydrothermal waters have significantly higher selenium to sulphate ratios than hydrothermal waters. The selenium to sulphate ratios allow to distinguish two different sulphur sources in the Aso catchment: hydrothermal waters impacted by the oxidation of sulphides or controlled by disproportionation reactions of SO2, and non-hydrothermal sulphate. Overall, the results show that hydrothermal waters strongly influence the sulphur budget, accounting for 67 to 91% of the total sulphate flux at the caldera outlet. The dissolution of magmatic CO2 and SO2 contribute to >60% of the observed weathering fluxes in this volcanic area. This suggests that magmatic gases and hydrothermal fluids should be considered for the estimation of biogeochemical budgets at the regional and global scales. However, the temporal variation of the magmatic activity and its effect on the chemistry of waters have to be investigated in future work.

Replenishment and mean residence time of root-zone water for woody plants growing on rocky outcrops in a subtropical karst critical zone

Quantifying root-zone water replenishment (β) is essential to understanding how root-zone water is partitioned and released, and the associated residence time of water in Earth’s critical zone. However, dynamics of such water replenishments and mean residence time in karst environments remain poorly understood. In this study, stable isotope compositions of precipitation and plant xylem water were investigated for eight woody plant species growing on two rocky outcrops in a subtropical karst critical zone. A new method was proposed for estimating mean residence time of root-zone water with a consideration of precipitation replenishment. The results show that all these species had seasonal water sources during the growing seasons, reflecting by seasonal variations in isotopic compositions of stem xylem water and β. Rapid increases in β were generally found for most species during the early wet season. The high values of β (>80%) appeared frequently during the main growing seasons accompanied with frequent precipitation events, indicating that precipitation frequency is particularly important for these plants growing on the outcrops. Meanwhile, precipitation was likely to bypass the root zone (β = 0) after several high replenishment events, which suggests that not all precipitation events evenly contribute to root-zone water. In addition, mean residence time of root-zone water varied seasonally and differed among species, with a range from 2 to 80 days for most species. The differences of mean residence time of root-zone water among these species during the main growing seasons may suggest that less competition exists among these species in such rocky outcrops. This study provides insights into understanding vegetation response to future changes in precipitation patterns, and ecohydrological dynamics of karst ecosystems.

Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers.

Summary The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial.We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2‐h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use.Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre‐drought levels within 3 wk after rewetting.These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Determining Water Isotope Compositions for the IAEA WICO and North West Villages, South Africa

Deuterium (2H) and oxygen 18 (18O) stable isotopes in water are the key indicators of hydrological and ecological patterns and processes. The water isotopes δ2H and δ18O have been employed widely as tracers in hydrological and ecological research, as they are integrated into geological and biological systems in a predictable manner. The aim of this study was to determine the water isotope composition of the International Atomic Energy Agency’s (IAEA) Water Isotope Inter-laboratory Comparison (WICO) samples and to determine the Local Meteoric Water Line for North West Province (NWP) villages in South Africa. The IAEA WICO 2020 samples were obtained from the IAEA, Vienna, and borehole water samples from selected villages in the North West province of South Africa were randomly collected to investigate the relationship between the stable isotopes (18O and 2H) and the climate in underground water aquifers. A cavity ring-down spectroscopy analyzer with laser-current-tuned cavity resonance, Picarro L2140-i, was used to measure triple water–isotope ratios with high precision. The IAEA WICO results obtained for the d-excess were in a satisfactory range and the margins of error were close to those required by the IAEA. The δ2H values ranged between −33.00‰ and −8.00‰, while the δ18O values ranged between −5.50‰ and −2.00‰. The results of this work have shown that our water science and management lab is capable of undertaking inter-laboratory comparisons for the IAEA.

Groundwater resource protection and spring restoration in Upper Jhelum Basin (UJB), western Himalayas

Groundwater is a significant source of fresh and clean water to millions of people living in the Upper Jhelum Basin (UJB) of Kashmir, India. During the past two to three decades the decline of water table in the shallow aquifers led to a reduction of spring discharge and/or drying up of some springs and wells. In the present study, hydrogeological investigation was carried out to understand the causes and factors for drying up or attenuation of springs in alluvial aquifer(s) of the UJB. Exhaustive field campaigns were conducted in 2017 to collect field data and water samples were collected from tubewells, springs and streams for stable water isotopic analysis. The rampant excavation of riverbed minerals (sand and boulders) using high-tech machinery has intercepted the alluvial aquifers, which exacerbated the problem of water-table decline. The similar stable water isotopic composition of groundwater and stream water samples at certain locations indicates a good hydraulic connection exists between groundwater and stream water. The stable water isotopic composition also suggests that streams act as point sources of recharge at many locations. The results indicate that besides climate change, the shrinking of recharge areas (by constructing roads along stream banks, converting small ponds into cropland, etc.) and unplanned pumping are the dominant factors responsible for the drying up of the springs and wells in alluvial aquifers. The findings suggest that the aquifers and the springs can be restored by artificial recharge and by plugging the intercepted aquifers along river beds by construction of masonry check dams across the streams near the affected sites. These findings can help in restoration of groundwater level and lost springs across the Himalayas at local and regional scales.

Validation of a coupled <i>δ</i><sup>2</sup>H<sub><i>n</i>-alkane</sub>–<i>δ</i><sup>18</sup>O<sub>sugar</sub> paleohygrometer approach based on a climate chamber experiment

Abstract. The hydrogen isotope composition of leaf-wax-derived biomarkers, e.g., long-chain n-alkanes (δ2Hn-alkane), is widely applied in paleoclimate. However, a direct reconstruction of the isotope composition of source water based on δ2Hn-alkane alone is challenging due to the enrichment of heavy isotopes during evaporation. The coupling of δ2Hn-alkane with δ18O of hemicellulose-derived sugars (δ18Osugar) has the potential to disentangle this limitation and additionally to allow relative humidity reconstructions. Here, we present δ2Hn-alkane as well as δ18Osugar results obtained from leaves of Eucalyptus globulus, Vicia faba, and Brassica oleracea, which grew under controlled conditions. We addressed the questions of (i) whether δ2Hn-alkane and δ18Osugar values allow reconstructions of leaf water isotope composition, (ii) how accurately the reconstructed leaf water isotope composition enables relative humidity (RH) reconstruction, and (iii) whether the coupling of δ2Hn-alkane and δ18Osugar enables a robust source water calculation. For all investigated species, the n-alkane n-C29 was most abundant and therefore used for compound-specific δ2H measurements. For Vicia faba, additionally the δ2H values of n-C31 could be evaluated robustly. Regarding hemicellulose-derived monosaccharides, arabinose and xylose were most abundant, and their δ18O values were therefore used to calculate weighted mean leaf δ18Osugar values. Both δ2Hn-alkane and δ18Osugar yielded significant correlations with δ2Hleaf water and δ18Oleaf water, respectively (r2=0.45 and 0.85, respectively; p<0.001, n=24). Mean fractionation factors between biomarkers and leaf water were found to be −156 ‰ (ranging from −133 ‰ to −192 ‰) for εn-alkane/leaf water and +27.3 ‰ (ranging from +23.0 ‰ to 32.3 ‰) for εsugar/leaf water, respectively. Modeled RHair values from a Craig–Gordon model using measured Tair, δ2Hleaf water and δ18Oleaf water as input correlate highly significantly with modeled RHair values (R2=0.84, p<0.001, RMSE = 6 %). When coupling δ2Hn-alkane and δ18Osugar values, the correlation of modeled RHair values with measured RHair values is weaker but still highly significant, with R2=0.54 (p<0.001, RMSE = 10 %). Finally, the reconstructed source water isotope composition (δ2Hs and δ18Os) as calculated from our coupled approach matches the source water in the climate chamber experiment (δ2Htank water and δ18Otank water). This highlights the great potential of the coupled δ2Hn-alkane–δ18Osugar paleohygrometer approach for paleoclimate and relative humidity reconstructions.