Isotopic Composition Of Water Research Articles

Triple oxygen isotopes in intertrappean fossil woods: Evidence of higher tropical rainfall during Deccan volcanism

Despite several attempts, the effects of volcanism and greenhouse gas emission on climate (e.g., rainfall) over India across the Cretaceous/Palaeogene Boundary (66.1 ± 0.01 Ma) remain a contentious issue. Inferences from conventional isotopic, palaeosol, floral analysis, and palaeoclimate models are contradictory ranging from much lower- to higher-than-today rainfall during this high CO2 atmosphere. To resolve this, robust tracer of triple oxygen isotopic ratios (ẟ18O-∆′17O) of silicified fossil woods from Deccan intertrappean lacustrine sediments, central India were analysed for retrieving the silicification temperature and ambient fluid (water) isotopic composition. Under certain assumptions and uncertainty, triple oxygen isotope data indicate that the fluids were derived from a mixture of meteoric and hydrothermal water. The meteoric water end member possibly had a low ẟ18OVSMOW value (∼− 12 ± 2‰) compared to the modern mean monsoon rainwater (∼− 5‰) which implies a substantially higher rainfall than today (∼1760 ± 200 to 1860 ± 200 mm/year) in the terminal Cretaceous India. This result supports the prediction of elevated precipitation by the recent new generation of global coupled models that consider various feedbacks and ocean dynamics, at a high CO2 atmosphere. It is also consistent with the paleo-botanical evidence but far exceeds the amount predicted by the older paleoclimate models pertaining to the Cretaceous period.

Hydrological, meteorological, and watershed controls on the water balance of thermokarst lakes between Inuvik and Tuktoyaktuk, Northwest Territories, Canada

Abstract. Thermokarst lake water balances are becoming increasingly vulnerable to change in the Arctic as air temperature increases and precipitation patterns shift. In the tundra uplands east of the Mackenzie Delta in the Northwest Territories, Canada, previous research has found that lakes responded non-uniformly to year-to-year changes in precipitation, suggesting that lake and watershed properties mediate the response of lakes to climate change. To investigate how lake and watershed properties and meteorological conditions influence the water balance of thermokarst lakes in this region, we sampled 25 lakes for isotope analysis five times in 2018, beginning before snowmelt on 1 May and sampling throughout the remainder of the ice-free season. Water isotope data were used to calculate the average isotope composition of lake source water (δI) and the ratio of evaporation to inflow (E/I). We identified four distinct water balance phases as lakes responded to seasonal shifts in meteorological conditions and hydrological processes. During the freshet phase from 1 May to 15 June, the median E/I ratio of lakes decreased from 0.20 to 0.13 in response to freshet runoff and limited evaporation due to lake ice presence that persisted for the duration of this phase. During the following warm, dry, and ice-free period from 15 June to 26 July, designated the evaporation phase, the median E/I ratio increased to 0.19. During the brief soil wetting phase, E/I ratios did not respond to rainfall between 26 July and 2 August, likely because watershed soils absorbed most of the precipitation which resulted in minimal runoff to lakes. The median E/I ratio decreased to 0.11 after a cool and rainy August, identified as the recharge phase. Throughout the sampling period, δI remained relatively stable and most lakes contained a greater amount of rainfall-sourced water than snow-sourced water, even after the freshet phase, due to snowmelt bypass. The range of average E/I ratios that we observed at lakes (0.00–0.43) was relatively narrow and low compared with thermokarst lakes in other regions, likely owing to the large ratio of watershed area to lake area (WA/LA), efficient preferential flow pathways for runoff, and a shorter ice-free season. Lakes with smaller WA/LA tended to have higher E/I ratios (R2 = 0.74). An empirical relationship between WA/LA and E/I was derived and used to predict the average E/I ratio of 7340 lakes in the region, which identified that these lakes are not vulnerable to desiccation, given that E/I ratios were < 0.33. If future permafrost thaw and warming cause less runoff to flow into lakes, we expect that lakes with a smaller WA/LA will be more influenced by increasing evaporation, while lakes with a larger WA/LA will be more resistant to lake-level drawdown. However under wetter conditions, lakes with a larger WA/LA will likely experience a greater increases in lake level and could be more susceptible to rapid drainage.

Caves demonstrate decrease in rainfall recharge of southwest Australian groundwater is unprecedented for the last 800 years

Billions of people worldwide rely on groundwater. As rainfall in many regions in the future is projected to decrease, it is critical to understand the impacts of climate change on groundwater recharge. The groundwater recharge response to a sustained decrease in rainfall across southwest Australia that began in the late 1960s was examined in seven modern speleothems and drip waters from four caves. These show a pronounced increase or uptick in regional drip water and speleothem oxygen isotopic composition (δ18O) that is not driven by a change in rainfall δ18O values, but is in response to the shallow karst aquifers becoming disconnected from rainfall recharge due to regional drying. Our findings imply that rainfall recharge to groundwater may no longer be reliably occurring in this region, which is highly dependent on groundwater resources. Examination of the longer speleothem record shows that this situation is unprecedented over the last 800 years.

Impact of Snowmelt Conditions on the Isotopic Composition of the Surface Waters of the Upper Ob River during the Flood Period

For many of the Siberian rivers, and the Upper Ob in particular, 70–80% of the volume of the annual water runoff is formed during the spring flood. Thus, factors influencing the formation of water runoff during the spring flood are paramount. We explain changes in the isotopic composition of the Upper Ob surface waters by changing different components’ contribution to the runoff water discharge over the spring flood period. We suggest estimating the time of meltwater flow from the Upper Ob watershed to the outlet section using the difference between the date of the complete melting of the snow cover in the catchment area and the date of the maximum light isotope composition of water in the outlet section. We show that a sharp short-term weighting of the isotopic composition of water in the river at the end of the first phase of the flood may be associated with the influx of autumn soil moisture, displaced from the soils by snowmelt waters.

Isotope Composition of Natural Water in Lake Onega Basin

In 2009–2018, the isotopic composition of oxygen and hydrogen in the atmospheric precipitation, groundwater and river and lake water of Lake Onega basin was studied. The weighted annual isotope composition of precipitation at Petrozavodsk was δ18O = −11.7‰ and δ2H = −84‰ and varied from −30.9 to −4.1‰ for δ18O and from −23 to −22‰ for δ2H. The isotopic composition of the water in Lake Onega was relatively uniform from −11.5 to −9.3‰ for δ18O and from −85 to −71‰ for δ2H. In the bays, the isotopic composition of the water varied more substantially than in the central part of the lake due to the river runoff during springtime flooding. In late summer, the concentrations of deuterium and oxygen-18 increased in the lake water, and figurative points on the δ2H vs. δ18O diagram shifted above the meteoric line. The absorption of the isotopically heavy summer precipitation and disequilibrium isotope fractionation during evaporation led to the enrichment of the lake water by heavy isotopes. Experiments were conducted to estimate the evaporation influence on the isotope enrichment of the residual water, and a comparison of the obtained isotope data with the experimental function showed that commonly, about 4% and up to 12% of water was lost during the spring and summer, respectively. In the water of the tributaries, the abundance of the deuterium and oxygen-18 varied in a wider scale than in the lakes, from −14.4 to −9.1‰ for δ18O and from −102 to −73‰ for δ2H. An evaporation loss of up to 35% was found for the rivers in late summer, and this value was proportional to the area of lakes and wetlands in the elementary watershed. The initial isotope composition of the water in the tributaries prior to evaporation was estimated to be δ18O ≈ −14.1‰ and δ2H ≈ −103‰ on average and crossed the approximation and meteoric lines. This estimation was close to the average composition of the groundwater, i.e., δ18O ≈ −13.4‰ and δ2H ≈ −94‰ on the Lake Onega catchment. The slightly increased isotope depletion of the calculated composition in the initial river water in comparison with the groundwater was the result of the contribution of the spring snowmelt water, which had a significant influence on the lake water balance.

Spatio-temporal variations in oxygen and deuterium isotope of different water sources in Sikkim Himalayas: An understanding towards regional scale basin hydrology

The glaciated Himalayan region is the third largest storehouse of freshwater in its frozen form that feeds several mega river systems. Especially the north-eastern Himalayas, where the snow and glacial stock depend on various moisture sources. This study presents a detailed investigation of water sources in the Teesta watershed in terms of oxygen (δ18O) and deuterium (δD/δ2H) values. The study helped understand the Spatio-temporal variability in the isotopic composition of water and moisture sources. Progressive depletion of δ18O values in the stream water as moving towards the non-renewable glacial source indicates orographic ascent attributed to cooling and subsequent rainout. This effect is profound in the case of the western tributary above Chungthang, the Lachen Chu river. The computed d-excess values suggest the dominance of mid-latitude westerlies in the high-altitude perennial streams of the west and north Sikkim. Also, a deterministic Monte Carlo-based three endmember δ18O-d-excess mixing model is developed to quantify the contribution of source endmembers such as snowmelt, rainfall, and glacial melt in the river streams and waterfalls of the Rathong river as well as Lachen Chu and Lachung Chu rivers above their confluence at Chungthang. With the model being iterated 10,000 times for each waterfall and stream sample, the results have been obtained at the mean, 25th, 75th and 95th percentiles. The model results in the mean glacial ice-melt contribution between 40% and 47% in different seasons, with the standard deviation between ± 3.5% and ± 10%.

Hydrogeochemistry and Isotopic Composition of Waters in the Renella Cave (Central Italy): New Insights into Groundwater Dynamics

This article presents data from monthly monitoring carried out on cave and stream waters belonging to the Renella Cave karst system from September 2020 to April 2022. Additionally, old data pertaining to cave waters from previous published work are discussed. The aim is to develop a dataset for future climatic and hydrological studies on the Renella Cave and its surface recharges. Water samples were collected and analyzed for major ions, δ18O and δ2H, on water molecules. The cave sump water level, water temperature, and electrical conductivity were continuously measured and compared to the precipitation data. Additionally, air temperature and pressure inside and outside the cave were also monitored. The classification based on the Piper-Hill diagram indicated that both stream and cave waters belong to the bicarbonate-alkaline earth hydrochemical facies, indicating that their chemism is essentially determined by the dissolution of the Grezzoni Fm. Isotopic data suggest that both stream and cave waters have a meteoric origin, and their variability mainly depends on underground homogenization. Hydrogeochemistry, isotopic composition, and the monitoring data allowed a deeper understanding of the cave karst system, and identified an underground circulation network that is fragmented into independent parts.

Spatiotemporal variability of stable isotopes in precipitation and stream water in a high elevation tropical catchment in the Central Andes of Colombia

AbstractThe Colombian Andean Mountains include the headwaters of the main basins of the country. However, the isotope composition of water in these high mountain ecosystems has been poorly studied. In this study, we analysed the first set of stable isotope data collected along a wide elevation range (2600–4950 m a.s.l.) in the Central Andes of Colombia. The stable isotope composition of stream water and precipitation was determined for a period between 2017 and 2018 in the Upper Claro River basin. The driving factors influencing the spatial and temporal variability of δ2H, δ18O, and d‐excess were identified, and compared with daily air temperature and precipitation data from seven meteorological stations. The local regression line was described by δ2H = 8.2 δ18O + 12.3, R2 = 0.98. The δ2H and δ18O values showed more depletion in heavy isotopes, and the d‐excess values were more negative during the rainy season. An altitude effect of −0.11‰/100 m and −0.18‰/100 m was estimated for stream water and precipitation δ18O values, respectively, with the latter showing non‐linear behaviour. The dataset was compared with Colombian stations of the Global Network of Isotopes in Precipitation database, and a back‐trajectory analysis of air masses was conducted and compared with the d‐excess values. The δ18O weighted mean values changed with respect to the position in the Central Andes, indicating contrasting altitude effects depending on the moisture sources. The most positive d‐excess values were attributed to moisture recycling enhanced by local ecosystem conditions and the origin of precipitation from the Amazon basin, which change during the year and across the northern Andes. The results showed a high level of variation because of differences in elevation, seasonality, and atmospheric circulation patterns during the year. This study contributes to knowledge of spatial and temporal isotope composition data in the northern Andes, delineation of water supply basins, and to the definition of ecosystem boundaries in the high mountains of Colombia.

Xylem water cryogenic vacuum extraction: Testing correction methods with CaviTron-based apple twig sampling

Cryogenic vacuum extraction (CVE), the most popular method for obtaining xylem water for isotopic analysis of ecosystems, is known to cause δ2H offset and may lead to biased inference on transpiration. While δ2H offset correction methods have been proposed recently, their accuracy in identifying plant water sources remains uncertain. In this study, capitalizing on the latest CaviTron centrifuge technique (CACT), we evaluated the accuracy of three current δ2H offset correction methods—CVE-bulk xylem water after the 8.1 ‰ correction method (8.1 ‰ Mean-Correc), CVE-bulk xylem water after the soil water isotope line correction method (Line-Correc), and CVE-bulk xylem water after the rehydration correction method (REHYDR)—for plant water source identification. We tested the three correction approaches on five apple (Malus pumila Mill) orchards. Our results revealed that although δ18O values of the CVE-bulk xylem water and the CACT-sap water matched, the δ2H values differed (P < 0.05), with a mean difference of −10.23‰ ± 3.75‰. By applying the available correction methods to the CVE-bulk xylem water, the δ2H offset was reduced to 2.13±3.75‰ (8.1 ‰ Mean-Correc), 4.43±4.48‰ (Line-Correc), and −0.44 ± 3.69‰ (REHYDR). The corrections also improved the Z-score. However, substantial δ2H offset variations remained after the corrections. Moreover, when applying the corrected xylem water isotopic compositions in plant water apportioning analysis, the Line-Correc method resulted in significant differences (P < 0.05) in 1 out of 80 cases, while the 8.1‰ Mean-Correc (0/80) and REHYDR (0/80) methods did not. Our results suggest that for the available correction methods for cryogenic extraction to reduce δ2H offsets, the REHYDR and 8.1‰ Mean-Correc methods outperformed the Line-Correc method in plant water source identification. Thus, we recommended that the REHYDR method and 8.1‰ Mean-Correc method be used to identify plant water sources of CVE-extracted bulk xylem water.

Strontium Isotope Characteristics (δ88/86Sr, 87Sr/86Sr) of Arima‐Type Brines Originated From Slab‐Fluids

AbstractIn the southwest Japan forearc, slab‐fluids produced from subducted materials migrate to crustal levels and appear as deep‐seated brine. We have analyzed for the first‐time stable strontium isotopes in non‐volcanic spring water with high salinity, referred to as Arima‐type saline water that likely originate from slab‐fluid that upwelled along major faults. The stable strontium isotope compositions of the saline water are isotopically light (δ88/86Sr = 0.122–0.157‰) and different from those of local bedrock and near‐surface water. The light strontium‐enriched and radiogenic signature of the saline water reflects the primary characteristic of slab‐fluids without an isotopic overprint in the crust. The Arima‐type brines show signatures of slab‐fluids at forearc depth, which is different from the slab‐fluids at subarc depth estimated from arc lavas. The characteristic features of the Arima‐type brines are explained by a larger contribution of subducted sediments and strontium isotope fractionation during fluids generation at shallower depth.

A shift of the prooxidant-antioxidant balance in the bodies of laboratory animals at five-fold increased deuterium content in drinking diet

This paper presents the results of the study of the effect of water with deuterium content of 750 ppm, used for simulation of the isotopic composition of water in ice caps at the poles of Mars, on oxidative processes in the liver tissue and blood of laboratory animals. It was found that prolonged consumption of deuterium-enriched water contributed to an increase in the deuterium content in blood plasma up to 487 ppm. As a result, increased antioxidant activity in the liver tissues and blood plasma was observed. In addition, the effect of a medium that contains 487 ppm of deuterium on the secondary structure of bovine serum albumin was also investigated in a model experiment. A decrease in intensity of circular dichroism and intrinsic tryptophan fluorescence spectra was found. This indicates that there are conformational changes in the structure of this protein at a time when the content of deuterium increases in the incubation medium. The results of our research point to the need to explore further the effect of drinking diet with the increased deuterium to (from 700 to 1000 ppm) on living systems, to explain the possibility for life on Mars.

Differences between stem and branch xylem water isotope composition in four tropical tree species

AbstractStable isotope studies (δ2H and δ18O) of water within plants are providing new information on water sources, competitive interactions and water use patterns under natural conditions. This is based on the assumption that there is no fractionation at the time of water uptake or during its transport within trees. However, previous studies have found fractionation does occur in water taken up through the roots in halophytic and xerophytic plants. It is unclear how widespread such fractionation is in other species. In this study, we tested this fractionation by comparing stem and branch xylem water isotopes over the period of one day (from 8 am to 5 pm) in four wet tropical rainforest tree species (Dendrocnide photinophylla,Aphananthe philippinensis,Daphnandra repandulaandMallotus polyadenos). We found branch water isotope ratios (δ2H and δ18O) were enriched compared with stem xylem water isotopes.D. photinophyllahad a significantly different branch δ18O ratio thanA. philippinensis,D. repandulaandM. polyadenos. In contrast, there were no significant differences in stem xylem δ18O among the four observed tree species. We found clear differences in the stable isotope ratios of δ18O for stem and branch xylem water forD. photinophyllaandD. repandulaof upto −0.85% and 0.50%, respectively. Remarkable δ2H differences were also found between stem and branch xylem water isotope ratios forA. philippinensis,D. repandulaandM. polyadenos, being upto −12.14%, −16.17% and −9.65%, respectively. A dual isotope (δ2H–δ18O) plot showed branch water values were more enriched than stem xylem water values forD. photinophyllaandD. repandula, indicating a clear difference between stem and branch xylem water. This study suggests that δ2H and δ18O fractionation could be a species‐specific phenomenon in tropical trees, which has important implications for plant water source identification and evapotranspiration partitioning.

Hydrogen isotopic evidence for nebular pre-hydration and the limited role of parent-body processes in CM chondrites

The conditions and environments in which hydrated phases in unequilibrated meteorites formed remain debated. Among carbonaceous chondrites, Mighei-type chondrites (CMs) display a large range in the degree of aqueous alteration, and thus record different stages of hydration and alteration. Here, we report the bulk H, C, and N contents, H and C isotopic compositions, and thermogravimetric signatures of the most- and least-altered CMs known so far, Kolang and Asuka 12236, respectively. We also report in-situ SIMS measurements of the hydrogen isotopic compositions of water in both chondrites. Compared to other CMs, Asuka 12236 has the lowest bulk water content (3.3 wt.% H2O) and the most D-rich water and bulk isotopic compositions (δD = 180‰ and 280‰, respectively). Combined with literature data, our results show that phyllosilicate-bearing CMs altered to varying degrees accreted water-ice grains with similar isotopic compositions. These results demonstrate that the hydrogen isotopic variations in CM chondrites (i) are not controlled by secondary alteration processes and (ii) were mostly shaped by interactions between the protoplanetary disk and the molecular cloud that episodically fed the disk over several million years. The minimally altered CM chondrites Paris and Asuka 12236 display peculiar, D-rich, hydrogen isotopic compositions that imply the presence of another H-bearing component in addition to insoluble organic matter and phyllosilicates. This component is most likely the hydrated amorphous silicates that are ubiquitous in these chondrites. CM bulk H and O isotopic compositions are linearly correlated, implying that (i) amorphous silicates in CM matrices were already hydrated by disk processes before the onset of CM parent-body alteration, and (ii) the quest for a hypothetically water-free CM3 is illusory.

Vacuum extraction of high-salinity water for the determination of oxygen and hydrogen isotopic compositions using cavity ring-down spectroscopy

Tracking the hydrogen and oxygen isotopic compositions of water samples can aid in understanding the water cycle. Compared to that of fresh water, analyzing the hydrogen and oxygen isotopic composition of high-salinity water is still a challenging task owing to the influence of dissolved salts. In this study, desalination was achieved by vacuum extraction, and the oxygen and hydrogen isotope ratios (δ18O and δ2H) were measured using cavity ring-down spectroscopy (CRDS). Different solutions of the individual salts (NaCl, KCl, MgCl2, and CaCl2) were prepared with deionized water (DI) having a known stable isotopic composition. After vacuum extraction, the maximum differences in isotopic composition was no more than 0.09‰ for δ18O and 0.30‰ for δ2H at 4 mol/L NaCl and KCl and 1 mol/L MgCl2 and CaCl2. The isotopic compositions of two high-salinity samples (IAPSO standard seawater and brine water) were determined by the proposed procedure as well as thermal conversion/elemental analyzer-isotope ratio mass spectrometry for comparison. The δ18O and δ2H results obtained using the two methods were consistent. The influence of isotope-salts was completely overcome by the proposed vacuum extraction desalination procedure. Thus, the method enables the rapid extraction of a large number of samples, regardless of the type and origin of the salt solutions, provided the concentrations of MgCl2 and CaCl2 are below 1.0 mol/L.

Water Age Dynamics in Plant Transpiration: The Effects of Climate Patterns and Rooting Depth

AbstractWater ages in plant transpiration reveal important processes of water dynamics in soil‐plant‐atmosphere continuum, which is regulated by water variability and availability in both precipitation inputs and root zone. However, the impacts of climate patterns and rooting depth on water age dynamics in transpiration are inadequately investigated. In this study, variations in root‐zone water replenishment are estimated based on stable isotope compositions of precipitation and plant xylem water, and a piecewise linear mixing water age model is employed to quantify water ages in plant transpiration under five different climates. The results show that water ages in transpiration are very dynamic in all climates with a range between 1 and 229 days. Water replenishments in the root‐zone reveal whether and how much precipitation enters the root zone. Climate patterns control median (mean) water age in plant transpiration but modified by rooting depth. In wet climates, plants mainly rely on water source that originates from precipitation in the current month (accounting for about 60% on average of their water sources), while plants in dry climates largely rely on water source that originates from precipitation fallen on previous months/seasons. These indicate that climate has profound impacts on plant water source by regulating precipitation inputs, root zone water storage and residence time, and evapotranspiration. Despite the limitations of this study, our study provides an alternative way to reveal the precipitation partitioning in root‐zone and seasonal changes in plant water use of precipitation, which highlights different vegetation responses to water variability between climate types.

The Isotopic Composition of Rainfall on a Subtropical Mountainous Island

Abstract Tropical islands are simultaneously some of the most biodiverse and vulnerable places on Earth. Water resources help maintain the delicate balance on which the ecosystems and the population of tropical islands rely. Hydrogen and oxygen isotope analyses are a powerful tool in the study of the water cycle on tropical islands, although the scarcity of long-term and high-frequency data makes interpretation challenging. Here, a new dataset is presented based on weekly collection of rainfall H and O isotopic composition on the island of O‘ahu, Hawai‘i, beginning from July 2019 and still ongoing. The data show considerable differences in isotopic ratios produced by different weather systems, with Kona lows and upper-level lows having the lowest δ2H and δ18O values, and trade-wind showers the highest. The data also show significant spatial variability, with some sites being characterized by higher isotope ratios than others. The amount effect is not observed consistently at all sites. Deuterium excess shows a marked seasonal cycle, which is attributed to the different origin and history of the air masses that are responsible for rainfall in the winter and summer months. The local meteoric water line and a comparison of this dataset with a long-term historical record illustrate strong interannual variability and the need to establish a long-term precipitation isotope monitoring network for Hawai‘i. Significance Statement The isotopic composition of water is often used in the study of island water resources, but the scarcity of high-frequency datasets makes the interpretation of data difficult. The purpose of this study is to investigate the isotopic composition of rainfall on a mountainous island in the subtropics. Based on weekly data collection on O‘ahu, Hawai‘i, the results improve our understanding of the isotopic composition of rainfall due to different weather systems, like trade-wind showers or cold fronts, as well as its spatial and temporal variability. These results could inform the interpretation of data from other mountainous islands in similar climate zones.

What Controls the Mesoscale Variations in Water Isotopic Composition Within Tropical Cyclones and Squall Lines? Cloud Resolving Model Simulations in Radiative‐Convective Equilibrium

AbstractWater isotopes are tracers of convective processes and are often used as proxies for past precipitation. These applications require a better understanding of the impact of convective processes on the isotopic composition of water vapor and precipitation. One way to advance this understanding is to analyze the isotopic mesoscale variations during organized convective systems such as tropical cyclones or squall lines. The goal of this study is to understand these isotopic mesoscale variations with particular attention to isotopic signals in near‐surface vapor and precipitation that may be present in observations and in paleoclimate proxies. With this aim, we run cloud resolving model simulations in radiative‐convective equilibrium in which rotation or wind shear is added, allowing us to simulate tropical cyclones or squall lines. The simulations capture the robust aspects of mesoscale isotopic variations in observed tropical cyclones and squall lines. We interpret these variations using a simple water budget model for the sub‐cloud layer of different parts of the domain. We find that rain evaporation and rain‐vapor diffusive exchanges are the main drivers of isotopic depletion within tropical cyclones and squall lines. Horizontal advection spreads isotopic anomalies, thus reshaping the mesoscale isotopic pattern. This study contributes to our understanding of mesoscale isotopic variability and provides physical arguments supporting the interpretation of paleoclimate isotopic archives in tropical regions in terms of past cyclonic activity.

Multi-species plantation intensifies soil water competition and groundwater depletion in a water-limited desert region

Multi-species plantation has long been applied in forestry and landscape restoration to provide better ecosystem services and functions. However, the differences in water-use patterns between mixed and pure plantations are poorly understood, especially in the arid/semi-arid artificial desert ecosystems. The stable isotope compositions (δ2H and δ18O) of rainwater, xylem water, soil water, and groundwater were analyzed over three growing seasons (2019–2021) to identify the water-use patterns of Artemisia desterorum Spreng (A. desterorum), Amorpha fruticosea (A. fruticosa) and Pinus sylvestris var. mongolica (P. sylvestris) in pure and mixed plantations in the Mu Us Sandy Land. Seasonally, there was a transference of water-use pattern from deep (moist layer and groundwater) to shallow water sources (active and stable layers) during the transition period between the dry and rainy season for both pure and mixed A. fruticosa and P. sylvestris, which was attributed to dimorphic root systems of arbor and shrub in the water-limited region. However, A. desterorum with shallower roots predominately absorbed water from the active and stable layers across the whole growing season regardless of planting pattern. Annually, the three plant species in pure stands extracted water predominantly from the stable layer (36.97%–46.16%), followed by the active layer (23.55%–36.98%). In the mixed stand, however, all the plants absorbed water mainly from the active layer (33.04%–39.44%), followed by the stable layer (30.46%–33.68%), suggesting that multi-species plantation intensifies shallow soil water competition. Additionally, the proportional contribution of groundwater to plant water uptake under mixed plantation was higher than the pure plantations, causing faster groundwater decline in the mixed stand (0.29 m yr−1) compared to pure stands (0.11–0.20 m yr−1). Our study demonstrates that multi-species plantation in the water-limited desert region intensifies soil water competition and groundwater depletion and thus adversely affects sustainable development goals.

Thallium isotope cycling between waters, particles, and sediments across a redox gradient

Thallium (Tl) isotopes are an emerging tool for tracking the history of molecular oxygen in seawater. Use of Tl isotopes in this manner requires a thorough understanding of the modern Tl isotope cycle, especially within the anoxic settings that typified Earth’s past oceans. To this end, we generated Tl concentration and isotope data for waters, particles, and sediments collected during three spring-summer-fall field seasons in Siders Pond, a salt-stratified and sulfide-rich (‘euxinic’) pond on Cape Cod (Massachusetts, USA). Over short timeframes (i.e., days to weeks), we observed a large degree of variability in the abundance (Tldiss = 9 pM–42 pM) and isotopic composition (ε205Tldiss = –3.9 ± 0.4; 2SD to –0.1 ± 0.7; 2SD) of dissolved Tl in pond surface waters. Thallium drawdown was common and favored removal of the lighter-mass Tl isotope. We surmise that biological Tl uptake plays a role in this phenomenon, but more work is warranted to confirm this hypothesis. Manganese oxides persisted in pond surface waters on many sampling days but had no obvious effect on Tldiss, only a slight effect on particulate ε205Tl values on the day of most prolific Mn oxide accumulation (driving ε205Tlpart as high as +0.9 ± 0.8; 2SD). In euxinic waters below the oxycline, rapid drawdown of dissolved Tl was observed on all sampling days together with a complimentary increase in particulate Tl (up to Tlpart = 21 pM). Strong associations between Tl and sulfide, and between Tl and other chalcophile metals (Mo and Cd), suggest that Tl is probably removed from euxinic waters in association with particulate sulfides, and seemingly with no resolvable isotopic fractionation effect despite non-quantitative Tl removal. The sediment data track the longer-term Tl cycle in the pond (i.e., across years) and reveal limited isotopic variability. Thallium isotope compositions leached from surface sediments match those predicted for contemporaneous waters, or nearly so, at all depths in the pond (above, within, and below the oxycline). Apparently, only a small fraction of the short-term Tl isotope variability found in the oxic surface waters of Siders Pond gets transferred to anoxic waters, and essentially none is transferred to sediments. Our results reaffirm the notion that Tl isotopes are uniquely capable of tracking long-term sedimentary Mn oxide burial – not mere Mn oxide formation. Our results also verify the ability of sediments formed under reducing conditions to capture the Tl isotope composition of contemporaneous waters. More broadly, the results of our investigation bolster our knowledge of the modern Tl isotope cycle and thereby permit more confident inferences of this cycle in Earth’s past.

Water sources used by artificial Salix psammophila in stands of different ages based on stable isotope analysis in northeastern Mu Us Sandy Land

Understanding the water use pattern of artificially restored vegetation is important to provide reasonable water management strategies. However, the characteristics, controls and differences of water sources of planted Salix psammophila at different developmental stages remain poorly understood. In this study, the hydrogen and oxygen isotope compositions of xylem water, soil water, and rainwater were measured and coupled with the MixSIAR model to investigate the variations and controls of water use patterns of different aged S. psammophila in a semi-arid region in northeastern Mu Us Sandy Land. Results showed that >60% of water used by 6-yr S. psammophila derived from 0 to 120 cm soil layer throughout the growing season due to the relatively shallow roots (0–100 cm). In contrast, water sources used by 12- and 18-yr S. psammophila differed markedly between the dry and rainy seasons, which were attributed to the developed dimorphic root systems and seasonal changes in soil water profile. In the dry season, 12- and 18-yr S. psammophila extracted 71.93% and 68.91% of water from 120 to 300 cm and 40–200 cm soil layer, respectively; in the rainy season, however, they both shifted main water sources to 0–120 cm soil layer (65.09% and 56.14%). The severe soil desiccation and dead roots observed in the 18-yr stand decreased the ability of old S. psammophila to extract water from the deep soil layer (200–300 cm) in the dry season. Thus, changes in the vertical distribution of roots and soil water contents controlled the seasonal water use pattern of different aged S. psammophila. Our study demonstrates that the age-dependent water use pattern of S. psammophila should be considered to implement the corresponding management measures in future afforestation plans to avoid overuse of water and maintain sustainable vegetation restoration in the water-limited desert regions.