Isotope Mass Balance Calculations Research Articles

Position-Specific Oxygen Isotope Analysis in Inositol Phosphates by Using Electrospray Ionization-Quadrupole-Orbitrap Mass Spectrometry.

Conventional isotope-ratio mass spectrometry measurements obscure position-specific isotope distributions in molecular compounds because these measurements require an initial step that converts compounds into simple gases by combustion or pyrolysis. Here, we used electrospray ionization (ESI)-based Orbitrap mass spectrometry to measure oxygen isotope ratios in the phosphate and hydroxyl moieties of inositol phosphates. A thermal hydrolysis experiment was conducted using 18O-labeled water to examine the position-specific oxygen isotope exchange in inositol hexakisphosphate (IP6) as well as its hydrolysis products IP5, IP3, and PO3 fragments. Measurement precisions of the position-specific and molecular-average oxygen isotope values of inositol phosphates were better than ±1.1‰ and ±0.5‰, respectively. Under optimized ionization and Orbitrap parameters, this level of precision was obtained within 30 min of run time at 60 μM initial concentration of inositol phosphate. The ability to measure phosphate-specific oxygen isotopes in inositol phosphate enabled the quantification of isotope exchange, which did not occur in phosphate on IP6, IP5, IP3, and PO3 fragments, meaning that the change in isotopes should have resulted from hydroxyls in the ring. Isotope mass balance calculations corroborated that hydroxyl oxygens are derived from 18O-labeled water. With the observed sensitivity and precision achieved in this study, Orbitrap IRMS proved to be a promising tool for investigating the position-specific oxygen isotopes in organophosphorus compounds. These outcomes open up numerous potential applications that can expand our understanding of phosphorus cycling in the environment.

Mg isotopes of siliciclastic sediments on continental marginal sea: Insights for the potential to trace silicate weathering

In the last decade, much attention has been devoted to Mg isotopic behavior during various weathering processes. Nonetheless, whether Mg isotopes in terrigenous siliciclastic sediments in modern oceans can reflect continental weathering regimes remains unresolved. Understanding the major controls of Mg isotope fractionation in siliciclastic sediments is thus a prerequisite for decoding past Mg isotope records for tracing continental weathering history. In this study, we present mineralogical, elemental, and Mg isotopic compositions of terrigenous siliciclastic sediments in the Changjiang (Yangtze River) estuary and the adjacent continental shelf of the East China Sea. δ26Mg values of the clay-sized sediments vary from −0.15‰ to 0.00‰ and lack of the correlation with the proportion of different clay minerals. Our results suggest that the incipient to intermediate weathering processes are characterized by the progressive leaching of isotopically light Mg from silicates, leading to the enrichment of 26Mg in secondary clay minerals. Therefore, Mg isotopic variability of clay-sized sediments is primarily determined by catchment weathering, rather than by provenance lithology or mixing of clay minerals. An isotopic mass balance calculation indicates that the δ26Mg value of silicate weathering flux on a continental scale is ca. -0.60‰ to −0.40‰. The new constraint on the δ26Mg value of silicate weathering flux provides a new opportunity to better understand the Mg cycle. The Mg flux from silicate weathering in the Changjiang catchment is estimated as ∼15–17 × 1010 mol/yr, accounting for ca. 50% to 60% of the total Mg influx of Changjiang River into the ocean. In addition, numerical modeling with the up-to-date flux and isotope fractionation data have indicated that a slight enhancement of silicate weathering can potentially drive the rapid rise of Mg/Ca in seawater since late Cenozoic, but maintain the constancy of seawater δ26Mg as reported by literature. Our study demonstrates that Mg isotopes of clay-sized sediments can be used to constrain the relevant δ26Mg value of silicate weathering flux on a continental scale, which is a key parameter to trace the Mg cycle on Earth's surface.

Hydrothermal origin of heavy potassium isotope compositions in altered oceanic crust: Implications for tracing the elemental cycle

Stable potassium isotopes are easily fractionated by fluid-rock interactions and provide a tool to probe the role of hydrothermal alteration of the oceanic crust in modifying the chemical composition of seawater and, through subduction, the deep Earth. An in-depth understanding of K isotope fractionation during high-temperature hydrothermal oceanic crust alteration is necessary to refine the use of K isotopes to track geochemical processes. This study investigated a suite of samples of altered upper oceanic crust, that formed at the East Pacific Rise and is exposed at the Hess Deep, to constrain K isotope fractionation by combining mineralogical, elemental, isotopic, and synchrotron-based spectroscopic approaches. Relative to the protolith, isotopically heavy K is enriched in the sheeted dike complex (δ41K of -0.48 ± 0.15 to 0.14 ± 0.13‰, ave. -0.22‰) where the rocks are generally depleted in K through high-temperature (hundreds of °C) fluid-rock reaction. In contrast, samples from the lava/dike boundary (-0.59 ± 0.14 to -0.48 ± 0.12‰, ave. -0.54‰) and lavas (-0.54 ± 0.10 to -0.42 ± 0.09‰, ave. -0.46‰), commonly exhibit net enrichment of K relative to the protolith due to low-temperature (tens of °C) fluid-rock reaction and have δ41K similar to fresh basalts. We interpret the observed isotopic variation in the sheeted dikes as being primarily due to one or both of the following: (i) diffusive K loss, with 39K diffusing faster than 41K, and (ii) neoformation of albitic plagioclase, which is identified as an important 41K host in altered sheeted dikes. Using isotope mass balance calculation, we show that isotopic fractionation occurring during high-temperature alteration may be difficult to identify in hydrothermal vent fluids because the δ41K of the K leached from the rocks is similar to that of the protolith. Hydrothermal K outputs to the ocean may not contribute significantly to elevating the δ41K of modern seawater relative to the silicate Earth. In contrast to upper oceanic crust altered at low-temperature and seafloor sediments, subducting ocean crust altered at high-temperature adds rocks with δ41K higher than the protolith to the mantle. Because the high δ41K value of the sheeted dikes is generated during the loss of K from these rocks, hence expressed most strongly in K-depleted samples, tracking this signature in arc magma and the mantle will be challenging.

No biological effect on magnesium isotope fractionation during stromatolite growth

The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ13C values, up to +20‰, and variable δ26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

Temporal and lake-specific variations in oxygen and hydrogen stable isotopes in a boreal lake-chain during two hydrologically differing years

ABSTRACT Hydrogen and oxygen stable isotope mass ratios (δ2H and δ18O) are conservative tracers that reflect hydrological conditions of watersheds and lakes, information that can be used for water budgets, such as estimating nutrient loading from the catchments and direct and indirect effects of climate change on waterbodies. We measured δ2H and δ18O values and estimated water mass balances over 2 hydrologically differing years in a chain of 10 boreal lakes. During a warm and dry summer, evaporative loss enriched the isotope values at lake surfaces. Thus, in lakes that were thermally stratified during summer, the isotope values differed distinctively between epilimnion and hypolimnion. During the following rainy summer, the within-lake variation of isotope values was much lower with no clear trend, presumably a result of a more unstable water column and faster water throughflow in the lake chain. After autumnal mixing the lake δ18O values were strongly correlated with lake area and proportion of inland waters of the catchment area. The interannual within-lake difference was then ∼1‰, indicating differences in evaporation. Isotope mass balance (IMB) calculations and those based on lake temperature and hydrological measurements in the nearby region yielded similar results for the rainy ice-free period. Presumably, prerequisites of steady-state conditions for IMB were better met in the rainy year than during the previous drier year. For some lakes the results suggested remarkable volumes of groundwater inflow, whose role in boreal lake water budgets deserves more attention in future studies.

Large-scale basement mobilization endows the giant Carlin-type gold mineralization in the Youjiang Basin, South China: Insights from mercury isotopes

The metal source and genesis of hydrothermal mercury-rich gold metallogenic systems occurring far away from active continental margins remain puzzling. The Youjiang Basin of South China, where exists numerous Carlin-type gold deposits and some synmineralization hidden intrusions, is a natural laboratory to address this issue due to it was up to 1000 km inward from the active continental margins of South China when mineralization. Here, we use mass-independent fractionation of mercury isotope ratios (reported as ∆199Hg), which is predominantly generated during Hg photochemical reactions on Earth’s surface and has superiority of isotopic inheritance during hydrothermal processes, to address the metal source of the Youjiang Carlin-type gold deposits. Ore-associated sulfides from seven representative deposits display negative to near-zero ∆199Hg values (−0.29‰ to 0.04‰), which fall in between that of the regional Precambrian basement rocks (−0.21‰ to 0.06‰) and deep magmatic-hydrothermal systems (∼0‰), suggesting a binary mixing of Hg from these two sources. An isotope mixing model and mass balance calculations demonstrate that ∼1000 km3 of the basement rocks, which contributed to 86% of Hg budget, were leached and remobilized by magmatic-hydrothermal fluids and deep-circulating crustal fluids to endow the gold reserves of these deposits. Given that traditional S, Pb, C, and O isotopic data yielded indirect and ambiguous constraints on metal source due to their complex evolution processes and isotope fractionation during the fluids ascended. Our results, therefore, highlight the great advantage of using Hg isotope as a new tracer to understand metal sources of hydrothermal deposits.

A New Simulation‐Optimization Framework for Estimation of Submarine Groundwater Discharge Based on Hydrodynamic Modeling and Isotopic Data

AbstractEstimation of submarine groundwater discharge (SGD) has traditionally relied on isotopic mass balance calculation, which is a form of regional box modeling. Here we propose an entirely new approach for SGD estimation by coupling a general marine hydrodynamic simulation model with a global optimization algorithm. The newly developed approach has the distinct advantage of being able to map the spatial distribution of SGD. The results of this study show that the estimated SGD exhibits a strong spatial heterogeneity. Furthermore, this study underscores the important effect of isotope sampling time on SGD estimation. Thus, the fluctuation of seawater level and a lag in isotope activity to water level changes should be considered to assure the representativeness of measured isotope activities. Overall, the simulation‐optimization framework adds a versatile and powerful new tool for more detailed SGD estimation.

Decoupling of physical and chemical erosion in the Himalayas revealed by radiogenic Ca isotopes

Determining the rate of CO2 consumption associated with the Himalayan uplift is an essential prerequisite to understanding climate evolution throughout the Cenozoic. The riverine fluxes of dissolved calcium can be used to quantify uptakes of atmospheric CO2 by chemical weathering, but this approach requires deciphering the silicate-derived Ca flux from the generally dominant carbonate-derived Ca flux. Here we present high-precision radiogenic calcium (40Ca) analyses of bank sediments and dissolved loads from a network of rivers in central Nepal Himalaya and Bangladesh, to constrain the sources and relative contributions of carbonate and silicate-derived calcium to the dissolved loads of Himalayan rivers. Calcium isotope analyses were performed in multidynamic mode by thermal ionization mass spectrometry, yielding an external precision of ±0.4 ε-units (2 S.D.). Our results show that silicate catchments exposed in the Himalayan range have variably radiogenic ε40Ca compositions relative to seawater, ranging from +0.7 in the TSS to +14 in the LH. In contrast, sedimentary carbonates, including metamorphosed dolomites with variably radiogenic 87Sr/86Sr, exhibit uniform ε40Ca identical to modern seawater. The homogeneous ε40Ca composition of sedimentary carbonates confirms the relative resistance of radiogenic Ca signatures to post-depositional alteration and provides a robust baseline against which the contribution of silicate weathering to the riverine Ca flux can be evaluated. Dissolved load compositions of rivers draining LH catchments exhibit ε40Ca values ranging from 0 to +11, reflecting the relative contribution of unradiogenic carbonates and highly radiogenic metapelitic units of the LH sequence. HHC rivers show a more moderate range, from 0.5 to +2, reflecting the less radiogenic composition of HHC paragneisses. Himalayan front rivers draining all three lithotectonic units of the Himalayan range exhibit moderately radiogenic ε40Ca, ranging from +0.5 to +1.4 and overall similar to the average composition of the Ganga mainstream in Bangladesh. Using an approach combining conventional alkalinity budgets with isotopic mass balance calculations, we show that the dissolved silicate Ca flux of major Himalayan front rivers is primarily derived from the weathering of metapelitic rocks of the lesser Himalaya. In contrast, regions of the high range that generate the bulk of the detrital flux to the Gangetic floodplain represent a subordinate source of silicate-derived Ca. This result demonstrates that the chemical weathering of Ca-silicates in the Himalayan system is decoupled from physical erosion. The extensive erosional activity observed in the high range may thus play a limited role in promoting CO2 consumption and global cooling by silicate weathering.

Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea

Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (δ30Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26–0.52 wt% or 92–185 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ∼900 μM. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 μM, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.

Carbon and oxygen isotopic composition of fluid inclusion CO2 and CH4 within the Archean Junction gold deposit, Kambalda, Western Australia

ABSTRACT The Junction orogenic gold deposit in the St Ives district of Western Australia is characterized by a syn-gold mineral assemblage dominated by quartz-calcite-albite-biotite-chlorite-pyrrhotite. The deposit had a light hydrocarbon,CO2, and O2 soil-gas signature above known mineralization prior to mining and it has been proposed that the source of the light hydrocarbon gases in the soil was gas-rich fluid inclusions trapped in gold-related alteration minerals, particularly calcite, albite, quartz, and pyrrhotite. Linking the soil gases with those in the deposit is extremely difficult. However, establishing that the gases in the soil are indeed present within the deposit and that those gases are related to the syn-Au alteration minerals is achievable through stable-isotope studies. Carbon and O stable-isotope compositions of pre-gold, syn-gold, and post-gold quartz veins; syn-gold and post-gold calcite; and CO2 and CH4 in the fluid inclusions that each of these minerals host were investigated to establish if the various mineral and fluid-gas species in the deposit are in isotopic equilibrium with each other, an important first step to relate syn-ore minerals with the relevant gases. Pre-ore Mo-type quartz veins contain CO2 (δ13Cgas = –1‰) and CH4 (δ13Cgas = ca. –33‰) in fluid inclusions at a ratio of ca. 93:7. The paucity of Mo-type quartz veins in the deposit suggests that these veins were not the main source of the soil-gas signature. Syn-gold alteration post-dates the Mo-type quartz veins. Quartz and co-existing calcite in the Au-bearing Junction shear zone have δ18Omineral values around 12.0 and 10.5‰, respectively. Multiple co-existing quartz-calcite pairs indicate that gold deposition occurred at ∼400 °C. This temperature agrees with mineral equilibria temperature estimates, the entrapment temperatures of fluid inclusions, and temperature modelling of solid-solution mineral phases. The temperature dictates that the quartz and calcite are in isotopic equilibrium with each other. The calcite in the Junction shear zone has δ13Cmineral values from –7.4 to –2.5‰, indicating that the CO2-rich ore fluid had a δ13Cfluid value of –3.7 ± 0.9‰. CO2 and CH4 released from quartz-hosted fluid inclusions have δ13Cgas values from –4.3 to +3.5‰ (mean = –1.5 ± 1.9‰) and –50.5 to –35.2‰, respectively. The isotopic composition of the fluid inclusion CO2 is in disequilibrium with co-existing CH4 that was co-released from the same quartz vein and the calculated δ13Cfluid value from co-existing calcite. Isotopic mass balance calculations using the two co-released gases show that the CO2 was initially in equilibrium with the syn-ore calcite but has since re-equilibrated with CH4 at temperatures below 200 °C. The abundance of CH4 in some quartz veins suggests that the syn-gold vein assemblage could be the source for the soil-gas anomaly. Post-gold veins contain quartz and calcite that have δ18Omineral values of ca. 11.0 and 10.0‰, respectively. Individual mineral pairs indicate precipitation at ∼320 °C from a fluid with a δ18Ofluid value of 4.7 ± 0.9‰, distinct from that which formed the syn-gold quartz veins. The post-gold calcite has δ13Ccalcite values from –7.5 to –5.4‰, indicative of formation from a CO2-bearing fluid having a δ13Cfluid value of –4.6 ± 0.9‰. The δ13Cfluid values are indistinguishable from fluid inclusion CO2 values of –3.6 ± 0.9‰, indicating no post entrapment re-equilibration, which suggests that CH4 was at trace volumes or absent in the post-gold quartz veins. These data lead to the conclusion that post-entrapment reequilibration between fluid inclusion CO2 and CH4 has occurred, but that the two gases were likely in equilibrium at the time of entrapment. This has implications for the interpretation of C isotope studies that focus on fluid inclusion CO2 measured from other gold and base-metal deposits, especially when the isotopic value of that CO2 is assumed to represent a specific source for the ore-forming fluids. The data also lead to a model that proposes that the syn-gold alteration assemblage could have produced the soil-gas anomalies observed above the mineralization.

Extensive Remineralization of Peatland‐Derived Dissolved Organic Carbon and Ocean Acidification in the Sunda Shelf Sea, Southeast Asia

AbstractSoutheast Asia is a hotspot of riverine export of terrigenous organic carbon to the ocean, accounting for ∼10% of the global land‐to‐ocean riverine flux of terrigenous dissolved organic carbon (tDOC). While anthropogenic disturbance is thought to have increased the tDOC loss from peatlands in Southeast Asia, the fate of this tDOC in the marine environment and the potential impacts of its remineralization on coastal ecosystems remain poorly understood. We collected a multi‐year biogeochemical time series in the central Sunda Shelf (Singapore Strait), where the seasonal reversal of ocean currents delivers water masses from the South China Sea first before (during Northeast Monsoon) and then after (during Southwest Monsoon) they have mixed with run‐off from peatlands on Sumatra. The concentration and stable isotope composition of DOC, and colored dissolved organic matter spectra, reveal a large input of tDOC to our site during Southwest Monsoon. Using isotope mass balance calculations, we show that 60%–70% of the original tDOC input is remineralized in the coastal waters of the Sunda Shelf, causing seasonal acidification. The persistent CO2 oversaturation drives a CO2 efflux of 2.4–4.9 mol m−2 yr−1 from the Singapore Strait, suggesting that a large proportion of the remineralized peatland tDOC is ultimately emitted to the atmosphere. However, incubation experiments show that the remaining 30%–40% tDOC exhibits surprisingly low lability to microbial and photochemical degradation, suggesting that up to 20%–30% of peatland tDOC might be relatively refractory and exported to the open ocean.

Feedback Between Carbon and Nitrogen Cycles During the Ediacaran Shuram Excursion

The middle Ediacaran Period records one of the deepest negative carbonate carbon isotope (δ13Ccarb) excursions in Earth history (termed the Shuram excursion). This excursion is argued by many to represent a large perturbation of the global carbon cycle. If true, this event may also have induced significant changes in the nitrogen cycle, because carbon and nitrogen are intimately coupled in the global ocean. However, the response of the nitrogen cycle to the Shuram excursion remains ambiguous. Here, we reported high resolution bulk nitrogen isotope (δ15N) and organic carbon isotope (δ13Corg) data from the upper Doushantuo Formation in two well-preserved sections (Jiulongwan and Xiangerwan) in South China. The Shuram-equivalent excursion is well developed in both localities, and our results show a synchronous decrease in δ15N across the event. This observation is further supported by bootstrapping simulations taking into account all published δ15N data from the Doushantuo Formation. Isotopic mass balance calculations suggest that the decrease in δ15N during the Shuram excursion is best explained by the reduction of isotopic fractionation associated with water column denitrification (εwd) in response to feedbacks between carbon and nitrogen cycling, which were modulated by changes in primary productivity and recycled nutrient elements through remineralization of organic matter. The study presented here thus offers a new perspective for coupled variations in carbon and nitrogen cycles and sheds new light on this critical time in Earth history.

Strong temporal and spatial variation of dissolved Cu isotope composition in acid mine drainage under contrasted hydrological conditions

Copper export and mobility in acid mine drainage are difficult to understand with conventional approaches. Within this context, Cu isotopes could be a powerful tool and here we have examined the relative abundance of dissolved (<0.22 μm) Cu isotopes (δ65Cu) in the Meca River which is an outlet of the Tharsis mine, one of the largest abandoned mines of the Iberian Pyrite Belt, Spain. We followed the chemical and isotopic composition of the upstream and downstream points of the catchment during a 24-h diel cycle. Additional δ65Cu values were obtained from the tributary stream, suspended matter (>0.22 μm) and bed sediments samples. Our goals were to 1) assess Cu sources variability at the upstream point under contrasted hydrological conditions and 2) investigate the conservative vs. non conservative Cu behavior along a stream. Average δ65Cu values varied from −0.47 to −0.08‰ (n = 9) upstream and from −0.63 to −0.31‰ downstream (n = 7) demonstrating that Cu isotopes are heterogeneous over the diel cycle and along the Meca River. During dry conditions, at the upstream point of the Meca River the Cu isotopic composition was heavier which is in agreement with the preferential release of heavy isotopes during the oxidative dissolution of primary sulfides. The more negative values obtained during high water flow are explained by the contribution of soil and waste deposit weathering. Finally, a comparison of upstream vs. downstream Cu isotope composition is consistent with a conservative behavior of Cu, and isotope mass balance calculations estimate that 87% of dissolved Cu detected downstream originate from the Tharsis mine outlet. These interpretations were supported by thermodynamic modelling and sediment characterization data (X-ray diffraction, Raman Spectroscopy). Overall, based on contrasted hydrological conditions (dry vs flooded), and taking the advantage of isotope insensitivity to dilution, the present work demonstrates the efficiency of using the Cu isotopes approach for tracing sources and processes in the AMD regions.

The respective roles of modern and paleo recharge to alluvium aquifers in continental rift basins: A case study from El Qaa plain, Sinai, Egypt

Even though assessment of aquifer characterization and recharge mechanisms in rift-related basins, especially in arid environments, is essential for developing sustainable management strategies and food security assurance, a little attention has been paid to understand these parameters. In this manuscript, we utilize an integrated remote sensing, hydrological and isotopic approach together with previously published data to better understand the aquifer characteristics and the respective roles of modern and paleo recharge to the post-Miocene alluvium aquifer in El Qaa plain, which represents the eastern margin of the Gulf of Suez continental rift basin. Our findings indicate that: (1) the alluvium aquifer is largely formed of sand and gravels intercalated with silt and clay lenses and exceeds 500 m thick, (2) the groundwater salinity gradually increases towards the Gulf of Suez (from 402 to 5613 mg/l) with increasing the distance from the crystalline rocks bounding the alluvium-dominated plain, (3) isotopic analysis reveals that all of the groundwater samples show mixed isotopic signature between modern and paleo waters (δ18O: −6.79 to −4.433‰, and δ2H: −46.36 to −21.99‰). Basin-scale hydrological parameters show that the aquifer receives an annual modern recharge of 39 × 106 m3 and isotopic mass balance calculations indicate that the modern water ranges between 20 and 70% of the total volume of water in the aquifer. We suggest three dam locations along the southernmost basins in El Qaa plain to enhance the modern recharge to the aquifer and to harvest surface runoff of an annual volume of 737 × 103 m3 sufficient to supply freshwater to 13 thousands of the residents of south Sinai. The present study suggests that topographically-driven modern recharge to rift-related basins is taking place with significant amounts even under hyper-arid conditions and calls for paying special attention to investigate the response of these aquifers to climate variability.

A rare earth element and Nd isotopic investigation into the provenance and deposition of the Dahongliutan banded iron formation and associated carbonates, NW China: Implications on Neoproterozoic seawater compositions

Rare earth element and Nd isotopic data for the recently discovered Neoproterozoic Dahongliutan banded iron formation (BIF) and associated carbonates are employed to infer aspects of the provenance and depositional environment, including ambient seawater compositions. The geochemical evidence is suggestive of deposition of the Dahongliutan marble in an epicontinental shallow–marine environment with riverine influx and low–T hydrothermal input, with restricted exchange to the open ocean. In contrast, the Dahongliutan BIF formed in a more open setting, with high–T hydrothermal input and old continental materials addition. The clearly distinct but negative εNd (0.593 Ga) values indicate that Neoproterozoic seawater at Dahongliutan could be spatially inhomogeneous with respect to 143Nd/144Nd ratios, owing to inefficient vertical and horizontal mixing of seawater closely related to the riverine Nd input derived from the weathering of different types and ages of continental crusts of the Western Kunlun orogenic belt and the Tarim Block. The Nd isotopic mass balance calculations of the Dahongliutan BIF indicate that most (>68%) of the Nd were derived from continental source materials transported by river water. In conjunction with previously published geochemical data of Neoproterozoic BIFs worldwide, we evaluate the chemical compositions of seawater where they formed, represented by a seawater reservoir strongly influenced by continentally derived freshwaters (>10%) mixed with open ocean and minor (<5%) high–T hydrothermal fluids as indicated by Sm/Yb and Eu/Sm ratios. By comparing the Nd isotopic features of Neoproterozoic BIFs, it is concluded that similar to modern oceans, the Neoproterozoic ocean was not well–mixed with respect to its Nd isotopic composition and continental source materials transported by river water dominated the great majority of Nd in Neoproterozoic seawater.

Groundwater Origin in Qanats, Chemo-Isotopic, and Hydrogeological Evidence.

A systematic study of the chemo-isotopic characteristics and origin of the groundwater was carried out at six major qanats in the hyper-arid Gonabad area, eastern Iran. These qanats as a sustainable groundwater extraction technology have a long history, supporting human life for more than a thousand years in this region. The Gonabad qanats are characterized by outlet electrical conductivity (EC) values of 750 to 3900 µS/cm and HCO3 -Na-Mg and Cl-Na water types. The Gonabad meteoric water line (Gn MWL) was drawn at the local scale as δ2 H = 6.32×δ18 O + 8.35 (with R2 = 0.90). It has a lower slope and intercept than the global meteoric water line due to different water vapor sources and isotope kinetic fractionation effects during precipitation in this arid region. The altitude effects on isotopic content of precipitation data were derived as δ18 O = (-0.0031 × H(m.a.s.l) )-1.3). The δ2 H and δ18 O isotopes signatures demonstrate a meteoric origin of the groundwater of these qanats. The shift of the qanat's water samples from the local meteoric water line (LMWL) in a dry period with higher temperatures is most probably due to evaporation during the infiltration process and water movement in qanat gallery. Based on the isotopic results and mass balance calculations, the qanats are locally recharged from an area between 2000 to 2400 m.a.s.l of nearby carbonate formations and coarse alluvial sediments. The dissolution of evaporate interlayers in Neogene deposits deteriorates the groundwater quality, especially in Baidokht qanat.

Silicon isotopes in Arctic and sub-Arctic glacial meltwaters: the role of subglacial weathering in the silicon cycle

Glacial environments play an important role in high-latitude marine nutrient cycling, potentially contributing significant fluxes of silicon (Si) to the polar oceans, either as dissolved silicon (DSi) or as dissolvable amorphous silica (ASi). Silicon is a key nutrient in promoting marine primary productivity, contributing to atmospheric CO2 removal. We present the current understanding of Si cycling in glacial systems, focusing on the Si isotope (δ30Si) composition of glacial meltwaters. We combine existing glacial δ30Si data with new measurements from 20 sub-Arctic glaciers, showing that glacial meltwaters consistently export isotopically light DSi compared with non-glacial rivers (+0.16‰ versus +1.38‰). Glacial δ30SiASi composition ranges from −0.05‰ to −0.86‰ but exhibits low seasonal variability. Silicon fluxes and δ30Si composition from glacial systems are not commonly included in global Si budgets and isotopic mass balance calculations at present. We discuss outstanding questions, including the formation mechanism of ASi and the export of glacial nutrients from fjords. Finally, we provide a contextual framework for the recent advances in our understanding of subglacial Si cycling and highlight critical research avenues for assessing potential future changes in these environments.

Sauna, sweat and science II – do we sweat what we drink?

ABSTRACTInspired by a previous ‘Sauna, sweat and science’ study [Zech et al. Isot Environ Health Stud. 2015;51(3):439–447] and out of curiosity and enthusiasm for stable isotope and sauna research we aimed at answering the question ‘do we sweat (isotopically) what we drink’? We, therefore, pulse-labelled five test persons in a sauna experiment with beverages that were 2H-enriched at about +25,600 ‰. Sweat samples were collected during six sauna rounds and the hydrogen isotope composition δ2Hsweat was determined using an isotope ratio mass spectrometer. Before pulse labelling, δ2Hsweat – reflecting by approximation body water – ranged from –32 to –22 ‰. This is ∼35 ‰ enriched compared to usual mid-European drinking water and can be explained with hydrogen-bearing food as well as with the respiratory loss of 2H-depleted vapour. The absence of a clearly detectable 2H pulse in sweat after pulse labelling and δ2Hsweat results of ≤+250 ‰ due to a fast 2H equilibration with body water are moreover a clearly negative answer to our research question also in a short-term consideration. Given that the recovery of the tracer based on an isotope mass balance calculation is clearly below 100 %, we finally answer the question ‘where did the rest of the tracer go?’

Use of carbon stable isotopes to monitor biostimulation and electron donor fate in chromium-contaminated groundwater

Hexavalent chromium Cr(VI) is a common inorganic contaminant in industrial areas and represents a serious threat to human health due its toxicity. Here we report experimental results from a field-scale investigation of Cr(VI) bio-immobilization at Hanford 100H reservation, a U.S Department of Energy facility (Washington State, USA). Microbial Cr(VI) reduction was stimulated via injection of a13C-labeled sodium lactate solution into the high-permeability aquifer consisting of gravel and coarse sand sediments. Concentrations and carbon isotope ratios of metabolites, including dissolved inorganic carbon and total organic carbon, and compound-specific analysis of acetate and propionate, together with phospholipid fatty acids (biomass) have been analyzed to help provide an understanding of the predominant redox processes accompanying Cr(VI) reduction. Results of our study indicate that the injection of an electron donor caused a sharp decrease of Cr(VI) concentration from ∼32 to ∼10 nM. Cr(VI) reduction was associated with a decrease in the concentration of carboxylic acids, such as lactate (∼6 mM to undetectable), propionate (∼9 mM to undetectable), and acetate (∼6 mM to undetectable), as well as dissolved inorganic carbon (30–10 mM C). Carbon isotope data indicate carbon transfers from the original substrate to organic byproducts and mineralized carbon. Concentrations of metabolites and stable isotope data as well as carbon isotope mass balance calculations were used to monitor biologically mediated reduction of Cr(VI).

Organic matter derived from kelp supports a large proportion of biomass in temperate rocky reef fish communities: Implications for ecosystem‐based management

Abstract The relative availability of alternative organic matter sources directly influences trophic interactions within ecological communities. As differences in trophic ecology can alter the productivity of communities, understanding spatial variability in trophic structure, and the drivers of variability, is vital for implementing effective ecosystem‐based management. Bulk stable isotope analysis (δ13C and δ15N) and mass balance calculations were used to examine patterns in the contribution of organic matter derived from macroalgae to food webs supporting temperate reef fish communities in two contrasting coastal waterways on the South Island of New Zealand: Fiordland and the Marlborough Sounds. Ten fish species common to both regions were compared, with up to 40% less organic matter from macroalgae supporting omnivorous species in the Marlborough Sounds. The largest differences in trophic position were found in those species exploited by fisheries. Furthermore, stratified surveys of abundance and species biomass combined with trophic position data were used to calculate regional differences in the contribution of macroalgae to whole fish communities in terms of density of biomass. In Fiordland, over 77% of the biomass of exploited reef fishes was supported by macroalgae, compared with 31% in the Marlborough Sounds. Surveys of macroalgal density and species composition in the two regions indicated that regional differences in trophodynamics may be explained by a lack of macroalgal inputs to the food web in the Marlborough Sounds. The findings demonstrate large regional differences in the incorporation of benthic and pelagic sources of organic matter to food webs supporting reef fish communities, highlighting the need for ecosystem‐based approaches to management to recognize spatial variability in primary production supporting coastal food webs.