Isotopic Balance Research Articles

Lacustrine groundwater discharge-derived carbon and nitrogen to regulate biogeochemical processes as revealed by stable isotope signals in a large shallow eutrophic lake

Eutrophic shallow lakes are hotspots of carbon (C) and nitrogen (N) accumulation and transformation, and are increasingly recognized as important sources of greenhouse gases (GHGs: CO2, CH4 and N2O). Lacustrine groundwater discharge (LGD) is a crucial component of the water budget and terrestrial material delivery for lakes, but its interplays with intrinsic CN biogeochemical processes remain less tackled. In this study, C and N ingredients and multiple stable isotopes (δ2H, δ18O, δ13C, and δ15N) were measured seasonally in groundwater, river water and lake water of a large eutrophic shallow lake in eastern China. The results revealed that groundwater is enriched with various forms of C and N that have similar sources and pathways as surface water in the lake and rivers. The isotope balance model also indicated that LGD derived C and N contribute significantly to lake inventories in addition to river runoff. These allochthonous C and N provide extra substrates for related biogeochemical processes, such as algae proliferation, organic matter degradation, methanogenesis and denitrification. Simultaneously, the excess oxygen consumption leads to depletion and hypoxia in the lake, further facilitating the processes of methanogenesis and denitrification. LGD functions not only as an external source of C and N that directly increases GHG saturations, but also as a mediator of internal CN pathways, which significantly affect hypoxia formation, GHG productions and emissions in the eutrophic lake. This study highlights the unrevealed potential regulation of LGD on biogeochemical processes in the eutrophic lake, and underscores the need for its consideration in environmental and ecological studies of lakes both regionally and globally.

Consistent solutions for simultaneous dynamic water, solute, stable isotope and radon balances to estimate average exchange fluxes between surface water bodies and groundwater

Many papers show that water, conservative solute, stable isotope and radon balances can be applied to the study of surface water – groundwater interaction. When undertaken simultaneously, such analysis provides an even more powerful method for estimating groundwater inflows and outflows, i.e. the exchange fluxes that for most people are hardest to estimate. The overarching objective of this paper is to provide practitioners with a consistent conceptual and simulation modelling framework with which to achieve a rapid initial understanding of surface water – groundwater interaction, as well as to guide the efficient and targeted acquisition of field data. This paper makes several important contributions. First, after an introduction that explains the complexities of groundwater flow patterns near surface water bodies, a set of general balance equations is developed using consistent terminology and notation. Second, after focusing on a simple conceptual model with steady groundwater inflow I, groundwater outflow O, precipitation P and ever-present evaporation E, analytical solutions are developed for transient and steady state conditions: one water balance solution is found to apply in all situations; solutions for the solute balance equation are developed for 23 different subcases; a transient balance equation for stable isotopes written in terms of 1+δ is found to apply in all situations; and a new transient solution for radon is found in terms of the upper incomplete gamma function. On the path towards developing the isotope balance equation, we present a rigorous approach, based on coupled nonlinear balance equations for abundant and rare isotope pairs (1H and 2H, then 16O and 18O), using isotope ratios and the fractions of each isotope. Third, we describe a Lake Water Balance Calculator (LWBC) written using GoldSim simulation software, a tool to help others to conduct the simultaneous analyses that we promote; we use GoldSim’s numerical methods to solve the general balance equations, we implement the analytical solutions to demonstrate that both GoldSim’s numerical methods and the analytical solutions are correct, and we also show that the 1+δ approximation is equivalent to the rigorous coupled equations, at least for heavy stable isotopes with very low abundance. Fourth, we describe field studies: we provide three examples of applications of LWBC to short-term studies of Perry Lakes East, Lake Jasper and Georgetown Billabong in Australia; we then describe WSIBal, a simulation model based on the same balance equations and describe its application to Lake Jasper over a 19-year period; finally, we describe R-WSIBal, which evolved from WSIBal, and explain its application to 2500 km of the Murray River in Australia.

CO2 flux from the French Massif Central groundwaters: Modelling and quantitative estimation of the degassing process

Passive rift systems are often characterized by CO2 degassing, witnessed by the presence of mineral and thermal springs, bubbling pools, mofetes. Despite these field manifestations, the quantitative estimation of the CO2 budget released to the atmosphere from these geodynamic structures is not well constrained. Here, we examine the chemistry of 169 springs, the isotopic composition of the dissolved carbon (δ13CTDIC) of 33 springs and the dissolved gases composition of 6 springs from the French Massif Central, part of the European Cenozoic Rift System (ECRIS), in order to describe the CO2 degassing process and to compute the CO2 emission rate released from groundwaters at regional scale. Water-gas-rock models reveal that the separation of gas from the liquid phase occurs at P-T conditions between 10 bar-180 °C and 1 bar-10 °C. The carbon mass and isotopic balance of spring waters of the French Massif Central allow us to compute a total deeply-sourced CO2 emission rate of 1.52 ± 0.14 × 109 mol yr−1, suggesting that the CO2 release from passive rift systems is significant at global scale and should be considered in the present-day global Earth degassing budget. The comparison of our data to other continental rift systems shows a high variability of CO2 emission rates, highlighting that more detailed studies are needed to constrain the CO2 flux from this geodynamic setting that, at present, is likely underestimated.

Stellarator–mirror fusion–fission hybrid – a fast route to clean and safe nuclear energy

The multiple-recycle fuel cycle for uranium-238 considered here, if practically realized, can bring revolutionary changes in nuclear energy. A full use of uranium-238 implies a practically infinite resource for power generation. Besides the energy, the fuel cycle net output is only fission products, which are co-products rather than waste. For the same amount of energy produced, the amount of fission products is two orders of magnitude less compared with the amount of spent nuclear fuel generated in currently exploited nuclear energy production scenarios. Using the simplest isotope balance model, key features of the multiple-recycle fuel cycle for uranium-238 are investigated. The repetition of this cycle results in smooth transformation of the initial fuel to ‘stationary’ fuel without strong variations in the fractional isotope content. Deficit of delayed neutrons is a threat of the fuel cycle considered as well as other fuel cycles that use plutonium. It has a dramatic impact on reactor controllability and safety. A solution to this threat could be a subcritical nuclear reactor with an external neutron source. In this paper, use of a stellarator–mirror (SM) fusion–fission hybrid for the multiple-recycle fuel cycle for uranium-238 is analysed. A summary of the experimental and theoretical studies on the SM hybrid is given. Preliminary results for principal design of a SM hybrid nuclear reactor for the multiple-recycle fuel cycle for uranium-238 are presented.

Investigating the hydrodynamic and biogeochemical evolutions of the hyporheic zone due to large-scale reservoir impoundment

The impact of large reservoir construction on the basin water cycle, ecological environment, and engineering safety needs to be studied urgently, especially, the evolution of the hyporheic zone in the dam site area connecting the basin SW-GW. Few hyporheic zone research investigates the evolution of SW-GW interaction in a certain period of large reservoirs. The Xiluodu Reservoir, located in the Yangtze River's upper reaches, has a dam height of 285.5 m and covers a basin area of 454,400 km2. The isotope balance calculation of water samples demonstrates that following impoundment, the upper boundary, lower boundary, and extension width of the hyporheic zone expands from 370–395 m, 320–330 m, and 125 m to 560–600 m, 370–390 m, and 200 m, respectively. Meanwhile, the upstream increment is bigger than downstream increment. Both temperature tracing and hydrodynamic methods show that the hyporheic flow velocity increases about three times during the initial impoundment cycle and then gradually fallbacks. The hydrochemistry of hyporheic zone water, which changed significantly, was strongly related to the season, depth of burial, and distance perpendicular to river but not to distance along river. The biological activities in the hyporheic zone strengthened after impoundment and gradually changed from the initial impoundment nitrification-dominated to denitrification-dominated. The SW-GW interaction shifts from alternating upwelling and downwelling to continuous downwelling according to the computation of increasing NO3- flux. The study helps better understand scientific and technical concerns such as the evolution of the hyporheic zone, river valley contraction, and dam foundation leakage in the Xiluodu reservoir. Meantime, the thoughts and approaches would also be useful to other large reservoirs.

Evaporation dominates the loss of plateau lake in Southwest China using water isotope balance assessment

The water balance budget in remote plateau lakes provides a fundamental information on the local climate-hydrological pattern. However, integrated investigation on the waters entering the lake, especially groundwater, was limited. To assess the current climate stress on Yunnan-Guizhou Plateau lakes, we collected rivers, groundwater, lake, and precipitation with varying isotopic compositions in the Chenghai Lake basin over four separate campaigns during a hydrological year. The wide and enriched variation of isotope composition in rivers, groundwater, and lake indicate that they have undergone distinct evaporations, which further reveal the recharging and mixing processes. Based on the similar isotopic signals between rivers and precipitation, rivers can serve as proxies for precipitation. Groundwater was primarily replenished by high mountain precipitation duo to the stable isotopic values in aquifers. Even through mass water in lake was able to smooth out some variability, the considerable isotopic variation of lake during the four collections suggested the influence of meteorological conditions. According to the assessment of isotope balance model, lake evaporation accounts for almost 65 % of the total inflow for one year, which partially explains the climate stress on the lake level. As the most sensitive variables, changes in relative humidity (h) and isotope composition of atmospheric moisture (δA) resulted in remarkable variations in E/I ratios and the constructed water isotope framework. These results shed light on the capacity of evaporation relative to lake input and provide interpretations on local paleoclimate and predicted-climate construction.

Widespread root-zone water bypass for various climates and species: Implications for the ecohydrological separation understanding

Root-zone water storage is a core component of many hydrological systems, and a critical variable influencing water transport in the soil-vegetation-atmosphere continuum and biogeochemical processes. However, the dynamics of water bypass and replenishment in root-zone during precipitation infiltration processes have been rarely investigated. Here we employed a piecewise isotope balance method to estimate the proportion of root-zone water replenished by precipitation, and conducted a meta-analysis to check the patterns of water bypass and replenishment in root-zone for various species across five climate zones and examined the potential influencing factors. The results show that high proportion of root-zone water replenishment is likely to appear during the transition period from dry season/soil to wet season/soil. Root-zone water bypass is widespread under various climates, which is likely to occur by downward drainage under wet soil condition or via evaporation loss under high atmospheric demand. Antecedent soil water condition and rooting depth are found to be the two key factors regulating dynamics of root-zone water bypass and replenishment. These findings provide useful insight into our understanding of ecohydrological modeling (e.g., non-well-mixed water pool in soils) and plant responses to precipitation pulses.

Isotope-based water balance assessment of open water wetlands across Alberta: Regional trends with emphasis on the oil sands region

Study regionWater sampling for stable isotopes (18O and 2H) was carried out during 2009–2019 across Alberta, Canada, as part of a survey targeting 1022 open water wetlands. The study presents the first site-specific wetland water balance assessment spanning Grassland, Parkland, Foothills, Mountains, and Boreal regions, including the oil sands region, a corridor of rapid development. Study focusClimate reanalysis, watershed data, and isotopic data for wetlands were incorporated into a steady-state isotope mass balance model, and applied to estimate site-specific evaporation losses from the wetlands, as well as runoff, groundwater inflow, total outflow, and seasonal drawdown. Regional-scale water balance fluxes, water balance indicators and site-specific classifications were mapped and described. New hydrological insights for the regionSystematic variations in evaporation losses, watershed runoff to wetlands, wetland discharge, and net groundwater inflow are revealed across the major subregions of Alberta including the oil sands region. Isotope balance calculations suggest that 13% of wetlands are predominantly evaporative, 87% have surface and/or groundwater outflow, 90% have positive water yields, 47% likely have groundwater inflow, and 2% are apparently fed by allochthonous water sources (either snow or glacial melt). For the 3 oil sands regions, a scoping survey suggests that between 20% and 40% of wetlands within the bitumen zones are detectably groundwater reliant compared to 35–50% in the wider vicinity of the deposits.

Using stable isotopes to track hydrological processes at an oil sands mine, Alberta, Canada

Study regionThis study was conducted at an oil sands operation in the Athabasca Oil Sands Region (AOSR), northeastern Alberta, Canada. The mine comprises open pit excavation of bituminous sands at two sites (Mildred Lake, ML, and Aurora North, AN), with a single hot-water extraction circuit connecting extraction plants at each mine. Study focusWater samples were collected and analyzed regularly over an eight-year period to establish inventories of site-wide water isotope signatures including seasonal and interannual changes in the recycle water circuit, and to permit future application of an isotope balance model to constrain poorly quantified processes such as evaporation losses, dewatering of tailings, and tailings pond connectivity of the recycle water circuit. New hydrological insights for the regionSampling of precipitation inputs over an 8-year period was used to constrain a local meteoric water line for the area. Differences in evaporative isotopic enrichment of tailings ponds at ML and AN are attributed to use of Athabasca River makeup water at the former site versus basal dewatering sources at the latter, with similar atmospheric controls at both. A conceptual model is developed summarizing temporal variations in water balance and isotopic signatures within the recycle water circuit, including accurate simulation of the unique isotopic enrichment of cooling tower blowdown. This study provides foundational evidence for application of stable isotope mass balance to monitor and improve industrial water use efficiency and management.

High-Ca vent fluids discharged from the Lutao arc volcanic hydrothermal system are associated with albitization and recycling of marine carbonate

The chemical and isotopic characteristics of calcium (Ca) in subduction zones are closely related to the budget of Ca and carbon cycles. Here we investigate the ultra-high Ca concentrations that characterize the hydrothermal fluids discharged from two types of vents, named the Zhudanqu brine vent (ZDQ) and the Huwaichi vapor spring (HWC), in the Lutao hydrothermal system at the north Luzon arc. The Ca concentrations of up to 159 mM and Ca/Cl ratio of up to 0.26 in the ZDQ vent fluids are possibly the highest ever reported for Ca enrichment in global seawater-circulated hydrothermal/geothermal systems. The differences in chemical compositions between the ZDQ and the HWC vent fluids are primary controlled by subcritical phase separation. The brine phase constitutes the ZDQ vent fluids, while the HWC vent fluids represent mixtures of the vapor phase and seawater. Both the vapor and the brine phases exhibit similar δ44/40Ca values (0.72 ± 0.05‰), suggesting no significant Ca isotope fractionation has occurred during phase separation.The hydrothermal endmember before phase separation (the “Lutao endmember”) presents depletions of 213 ± 15 mM of Na, 24.4 ± 0.4 mM of SO42−, and 10.2 mM of K, and enrichment of 130.2 ± 5.5 mM of Ca with respect to the percolated seawater. The total gained Ca is 154.6 ± 5.9 mM with a δ44/40Ca value of 0.67‰ – 0.77‰ (0.72 ± 0.05‰), considering anhydrite precipitation during hydrothermal circulation. The Holocene raised coral reef is unlikely to contribute substantial Ca into the Lutao system. Much of the gained Ca (111.6 ± 7.5 mM) is produced by high-degree albitization of the Lutao host rock, which is promoted by the low water/rock ratio (~ 2), slightly alkaline conditions, and relatively lower temperature of the Lutao system with respect to most mid-ocean ridge hydrothermal systems. Ca derived from this process inherits the Ca isotopes of plagioclase in the Lutao host rocks (δ44/40Ca = 0.82 ± 0.06‰). According to mass and isotopic balances, the recycled marine carbonate is proposed to contribute 43 ± 13.4 mM Ca with a δ44/40Ca value of 0.46−0.63+0.35‰ into the Lutao system. Such isotopically lighter Ca is derived from either pore fluids expulsed from underlying Philippine Sea sediments, or more probably, carbonate-bearing subduction fluids from the subducting South China Sea sediments and slab. The carbonate solubility in the subduction fluids could maintain at 600 mM near the reaction zone. The carbonate-rich fluids were subsequently migrated into the Lutao reaction zone and released an additional 43 ± 13.4 mM Ca via dolomitization. A small amount (~ 9%) addition of carbonate-rich fluids would not significantly change the budgets of Na, Mg, and Cl but could generate substantial Ca enrichment and Ca isotopic variations.

Preliminary study of water origin in the Uspallata valley and its contribution to the Mendoza river with isotopic and hydrochemical techniques. Mendoza, Argentina

The Uspallata Valley is located in Mendoza Province (Argentina), at an average altitude of 1,900 m asl. With a north-south direction and a length of around 20 km, it is an intermountain valley placed between the Pre-cordillera (to the east) and the Frontal Cordillera (to the west). The valley discharges to the Mendoza River, the main Andean course that irrigates the largest socio-productive oasis in Mendoza Province. For the last 20 years there has been a significant increase in urban growth and a disordered development of agricultural, touristic, and real estate activities in the southern part of the valley. The objective of this study is to use isotopic and hydrochemical techniques to develop a geohydrological conceptual model, in order to contribute to decision making processes of the provincial management that deals with water and land-use planning. Hydrochemical and isotopic techniques have been used to understand groundwater characteristics and provenance along the valley. Beyond compiling the scarce previous existing data, two monitoring surveys were carried out where physical-chemical parameters and stable isotopes were measured in 39 sites (wells, springs, creeks, and the Mendoza River). The preliminary isotopic balances performed suggest that around 70% of the groundwater comes from the Cordillera Frontal. Moreover, the integrated analysis of new data and existing geological and geoelectric information has allowed the development of a preliminary conceptual model identifying three zones relating to the origin of the water along the Frontal Cordillera. The study is part of the project “Use of isotopic techniques for improving the management of groundwater resources” ARG/7/008, which is funded by the International Atomic Energy.

Effect of snowmelt on the dynamics, isotopic and chemical composition of runoff in mature and regenerated forested catchments

Using isotopic and hydrochemical tools, we focused our study on water and element fluxes with snowmelt in two headwater catchments with different forest stands. One catchment is with mature, and the other catchment is with forest regenerating after a tree dieback. Sampling and analysis of the surface water and precipitation throughout one hydrological year, and of snowpack in snow season, enabled us to estimate the isotopic balance and chemical snowpack evolution, but also the snowmelt contribution to inlets and outlets of lakes. Stable isotope values varied from snowpack formation to snowmelt with δ2H amplitudes of −25‰ in the mature and −17‰ in the regenerating forest catchments over the duration of the snowpack persistence. The mature forest had one month longer duration of snow cover and higher concentration of solutes in the throughfall and snowpack. In both catchments, heavier isotopes of the water molecule (18O and 2H) preferentially left the snowpack, which stored considerable amount of rainwater and snowmelt. This resulted in heavy isotopes depletion in the end of the spring snowmelt. Ions were also eluted from the snowpack during rain events and partial snow melting throughout the winter, causing fluxes of diluted water at the end of the snowmelt. Our results demonstrate the hydrological and hydrochemical variability of the snowpack, which in the future may even increase with rising temperatures and changes of precipitation patterns.

Deep lake water balance by dual water isotopes in Yungui Plateau, southwest China

Stable isotopes of water (δ18O and δ2H) serve as a practical tool in the understanding of regional hydrological cycle, in particular, where the regional water cycle is stressed by both climate change and human activities. Lake water in southwest China is believed to play an important role in the local hydrology and possible modification of local climate. Water isotopes in lake systems are influenced by both the input water isotope and surface evaporation, such that they can be used as a natural tracer to evaluate these effects. Here we present a detailed water isotope balance study in a plateau lake – Fuxian Lake, the deepest and largest water body in Yungui Plateau, southwest China, with analogues of a steady state of water isotope equilibrium. The lake water isotopes are significantly enriched in heavy isotope due to evapoconcentration during the long residence time. We simulated the dual isotopes in the lake water using the Craig and Gordon model constrained by the observed water isotopes in lake water, river water, and atmospheric vapor. Results show consistent results for both δ18O and d-excess with values of −2.6‰ and −9.0‰ at the present relative humidity and E/I ratio. The test simulations also demonstrate how potential change in climate and/or lake water E/I ratio could impact lake water isotopes, and how the dual water isotope approach can unequivocally define the climatic and hydrological controls on lake water isotopes. These findings may be relevant for anticipating and detecting hydrological response to future climate change and water source management, e. g. inter - basin diversion projects. The results from this study highlight the potential of water isotopes in regional water cycle work, paleo climate reconstruction from lacustrine record, as well as human activity on water management.

Sulfide S and Pb Isotopic Constraint on the Genesis of Diyanqinamu Mo-Pb-Zn Polymetallic Deposit, Inner Mongolia, China

The Great Hinggan Range (GHR) hosts many large Mo deposits and vein-type Pb-Zn deposits and is one of the most important polymetallic metallogenic belts in China. Although Mo and Pb-Zn deposits are locally closely related in space in the GHR, it is disputed whether the Mo and Pb-Zn deposits have a genetic relationship. The Diyanqinamu Mo deposit located at the middle part of the northern GHR is a Late Jurassic large porphyry Mo deposit and closely adjacent by vein-type Pb-Zn deposit. In this work, we discussed the relationship between Mo and Pb-Zn deposits in Diyanqinamu mine based on the data of S and Pb isotopic geochemistry and geological information. In this mine, the Mo deposit is concentrated in the southern area with a distance of 500 m to the vein Pb-Zn deposit. The δ34SCDT values of the galena and sphalerite from the Mo deposit range from +1.73‰ to +7.29‰ with average of +5.04‰. By contrast, δ34SCDT values of the galena and sphalerite from the Pb-Zn deposit, ranging from +2.38‰ to +5.46‰ with average of +4.04‰, is similar to that of the Mo deposit. The formation temperatures of the Pb-Zn deposit calculated based on the sulfur isotope balance fractionation between sphalerite and co-existed galena range from 220 °C to 315 °C (average 247 °C), which is lower than that of the Mo mineralization (292–510°C). Pb isotopic results show that the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the Pb-Zn deposit range from 18.326–18.364, 15.541–15.589, and 38.054–38.214, respectively, which are slightly higher than those of the Mo deposit ranging from 18.287–18.331, 15.532–15.569, and 38.034–38.139, respectively. In the 206Pb/204Pb˗207Pb/204Pb diagram, sulfides sampled from the Mo and Pb-Zn deposits overlapped with each other and formed a linear distribution, indicating that they are derived from a mixed metal source with more external contribution to the Pb-Zn mineralization. This mixed signal is further confirmed by the geologic facts that the host rocks of the vein-type Pb-Zn deposit have abnormally high contents of Pb, Zn, and Ag, and experienced strong hydrothermal alteration. Combined with the ore geology, mineral assemblage, and isotopic geochemistry of the two types of mineralization, we propose that the Mo and Pb-Zn deposits in the Diyanqinamu mine represent different faces of the same porphyry system. This Mo-Pb-Zn metallogenic system would provide important clues on further prospecting of Mo and Pb-Zn resources in the GHR.

Potassium isotope fractionation during continental weathering and implications for global K isotopic balance

Potassium isotopic compositions of profiles through saprolites developed on a diabase in South Carolina, U.S.A., and on a granite in Guangdong, China, allow characterization of the behavior of K isotopes during continental weathering. Saprolites from the diabase profile are heavily weathered with chemical index of alteration (CIA) values up to 95; however, their K isotopic variation is limited, with δ41K ranging from −0.475 ± 0.028‰ in the unweathered diabase to −0.407 ± 0.021‰ for the saprolites. The lack of significant K isotope fractionation mainly reflects the conservative behavior of K in the diabase weathering profile, with >50% of the original K remaining in the saprolites. By contrast, K isotopes are fractionated during granite weathering and correlate with sample depth, CIA, and kaolinite abundance, with δ41K decreasing from −0.493 ± 0.030‰ in the unweathered granites at the bottom to −0.628 ± 0.021‰ in the most weathered saprolite close to the surface. These observations suggest the preference of light K isotopes in saprolites relative to fluids, which is further supported by the overall isotopically heavy nearby stream water samples (δ41K = −0.709 ± 0.017 to −0.339 ± 0.018‰). These results demonstrate that continental weathering plays an important role in the global K isotopic budget through the formation of isotopically heterogeneous rivers and weathered regolith. Recycling of K-rich crustal materials with distinct K isotopic signatures may produce distinct mantle K isotopic end members.

Tracing deep fluid source contribution to groundwater in an active seismic area (central Italy): A combined geothermometric and isotopic (δ13C) perspective

An understanding of the origin and migration pathways of geofluids in an active seismic area is of paramount importance in terms of societal challenges such as mitigation of seismic hazards. This study investigates the relationship between the stable isotope ratio 13C/12C of deep CO2 and geothermometry in selected groundwater samples located close to extensional faults in central Italy. The temperature range is inferred from an equation based on the Na/Li ratio and chemical geothermal modelling: 113–130 °C. Globally, the calculated temperature range agrees with that resulting from deep boreholes in the Northern Apennines. An alternative method is also included to better evaluate the difference in isotopic composition between the aqueous and gaseous δ13C(CO2) at deep condition. A review of previously published data shows that monitoring of the isotopic composition of the dissolved inorganic carbon (DIC) in springs fed by meteoric origin located in seismically active areas must take into account mass and isotopic balance to correctly evaluate the component, and the role, of deep fluid during seismic events. In particular, the coupled (and corrected) isotope and geothermometrical monitoring of the springs could help to distinguish between deep gas or deep fluid contributions to shallow aquifers. The results of this study indicate that faults play a crucial role in controlling the migration of crustal fluids. In addition, they reveal that possible evaluation of potential seismic precursors mandatorily requires a long period of monthly monitoring.

Reconstruction of Thermal Conditions in the Subboreal Inferred from Isotopic Studies of Groundwater and Calcareous Tufa from the Spring Mire Cupola in Wardzyń (Central Poland)

Studies with application of stable isotopes of oxygen and carbon have been performed on calcareous tufa, groundwater and dissolved inorganic carbon (DIC) from the spring mire cupola in Wardzyń. This study was focused on the verification of the a priori hypothesis that the analysed calcareous tufa is a chemical deposit and on the attempt to supplement an earlier scenario of environmental changes in the Subboreal with oscillations of water temperature. The constructed model of chemical and isotope balance, and δ13C determinations in DIC, allowed for calculating ratios of stable isotopes of carbon in particular speciations and in gaseous CO2. The obtained results coupled with δ13C values in calcite indicate that this mineral precipitated from the solution chemically (without the contribution of living organisms). Additionally, it was possible to reconstruct the temperature range at which the calcareous tufa was formed. The reconstructed scenario of changes in the thermal conditions was refined based on δ18O determinations in groundwater and calcite. Accordingly, the oldest calcareous tufa, with an age of about 5500 cal years BP, was formed in cool climate conditions (with average annual temperatures by about 3 °C lower than presently). The formation of younger series of the calcareous tufa took place between 4400–2900 cal years BP and represents a much warmer period with two distinct cooler episodes at 3900 and 3000 cal years BP, respectively. The course of the obtained temperature curves correlates well with the GISP2 curve and curves obtained for other sites in Northern, and Central Europe.

Vanadium cycling in the Western Arctic Ocean is influenced by shelf-basin connectivity

Water in the western Arctic Ocean tends to show lower dissolved vanadium concentrations than profiles observed elsewhere in the open ocean. Dissolved V in Pacific-derived basin waters was depleted by approximately 15–30% from the effective Pacific Ocean endmember. The depletion originates on western Arctic shelves and is not a result of mixing with a water mass with low V. While biological uptake may account for some of the V removal from the water column, adsorption onto particulate Fe is likely the dominant factor in removing V from shelf waters to the sediments. Once in the sediments, reduction should result in sequestering the V while Fe (and Mn) can be remobilized. A similar Fe-shuttling mechanism for V was previously described for the Peru margin (Scholz et al. 2011). Off the shelves, particulate Mn concentrations often exceed particulate Fe concentrations and thus may exert greater control on the V distribution in basin waters. Nonetheless, particulate V concentrations are much lower in basin waters and dissolved V thus behaves largely conservatively away from the shelf environment. Dissolved V concentrations in Atlantic-derived and Arctic deep waters were as much as 5 nmol/kg lower than those observed in deep waters of other ocean basins. The uniformity in deep water dissolved V between the sampled basins suggests that slow removal of V from the deep basins is probably not a factor in the deep water depletion. Vanadium-depleted incoming Atlantic waters (i.e., the source of Arctic deep waters) and/or removal of vanadium from incoming waters that pass over the shelves probably accounts for the deep water dissolved V depletion. Overall, our results demonstrate the utility of the V distribution as an additional tool to help understand the Arctic marine system. Furthermore, our work is pertinent to questions related to the net effect of marginal basin shelves on oceanic vanadium cycling, its isotopic balance, and how climate-induced changes in shelf biogeochemical cycling will impact vanadium cycling.

Root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate

AbstractThe root‐zone moisture replenishment mechanisms are key unknowns required to understand soil hydrological processes and water sources used by plants. Temporal patterns of root‐zone moisture replenishment reflect wetting events that contribute to plant growth and survival and to catchment water yield. In this study, stable oxygen and hydrogen isotopes of twigs and throughfall were continuously monitored to characterize the seasonal variations of the root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate in South Australia. The two studied hillslopes (the north‐facing slope [NFS] and the south‐facing slope [SFS]) had different environmental conditions with opposite aspects. The twig and throughfall samples were collected every ~20 days over 1 year on both hillslopes. The root‐zone moisture replenishment, defined as percentage of newly replenished root‐zone moisture as a complement to antecedent moisture for plant use, calculated by an isotope balance model, was about zero (±25% for the NFS and ± 15% for the SFS) at the end of the wet season (October), increased to almost 100% (±26% for the NFS and ± 29% for the SFS) after the dry season (April and May), then decreased close to zero (±24% for the NFS and ± 28% for the SFS) in the middle of the following wet season (August). This seasonal pattern of root‐zone moisture replenishment suggests that the very first rainfall events of the wet season were significant for soil moisture replenishment and supported the plants over wet and subsequent dry seasons, and that NFS completed replenishment over a longer time than SFS in the wet season and depleted the root zone moisture quicker in the dry season. The stable oxygen isotope composition of the intraevent samples and twigs further confirms that rain water in the late wet season contributed little to root‐zone moisture. This study highlights the significant role of the very first rain events in the early wet season for ecosystem and provides insights to understanding ecohydrological separation, catchment water yield, and vegetation response to climate changes.

Examination of the ecohydrological separation hypothesis in a humid subtropical area: Comparison of three methods

The ecohydrological separation between soil water sources for plant water uptake and groundwater recharge has been recently examined in various climate zones primarily based on isotopic composition of water. The existence of the ecohydrological separation has profound implications for mechanistic ecohydrological modeling and water resource management. However, it is still unclear when and where the ecohydrological separation occurs, especially in humid regions. In this study, high frequency sampling of precipitation, bulk soil water, groundwater and twig xylem water for hydrogen and oxygen isotope composition measurement was conducted in a humid subtropical site in the central southern China from March 2017 to April 2018. We examined evidence of the ecohydrological separation with three methods (dual-isotope space, line-conditioned excess (lc-excess), and the piecewise isotope balance (PIB) method). The results show that the isotopic composition of plant xylem and bulk soil water are not distinguishable from those of precipitation water on the dual-isotope space due to a weak evaporation effect at the study site, indicating that there is no evidence of the ecohydrological separation. However, the other two methods support the ecohydrological separation in this humid area, with the results from the PIB method revealing more temporal details. The present study suggests that the ecohydrological separation can happen in subtropical humid climate. It is more likely to occur in spring and winter at the study site when plant-accessible water pool has been replenished by antecedent precipitation, while ecohydrological connection seems to occur during winter snowmelt. With the limitations of three methods, the caution should be taken when only one method is applied in examining the ecohydrological separation in such an environment.