Experimental and simulation results reveal that this ultrahigh scaling-resistance is attributed to the combination of hBN’s atomically-smooth surface, in-plane atomic energy corrugation due to the polar boron-nitrogen bond, and the close match between its interatomic spacing and the size of water molecules. The latter two properties lead to strong polar interactions with water and hence the formation of a dense hydration layer, which strongly hinders the approach of mineral ions and crystals, decreasing both surface heterogeneous nucleation and crystal attachment.Formation of mineral scale on a material surface has profound impact on a wide range of natural processes as well as industrial applications. However, how specific material surface characteristics affect the mineral-surface interactions and subsequent mineral scale formation is not well understood. Here we report the superior resistance of hexagonal boron nitride ( Scale formation may have detrimental effects on the properties and functions of materials’ surfaces. Here the authors report the high scaling resistance of hexagonal boron nitride and relate it to the atomic level structure and interaction with water molecules. It is reported that economical loss due to mineral scaling in boilers, turbines, and heat exchangers accounts for 0.17–0.25% of the gross domestic product (GDP) in industrialized nations5. Understanding mineral scaling behavior is important for the development of next generation materials and technologies that address these critical challenges.Interfacial interactions play a fundamental role in many aqueous processes including adsorption, catalytic reaction, corrosion, filtration, and scale formation. In particular, scale formation, i.e., the development of mineral deposits on a material surface due to precipitation from the bulk solution and/or crystal formation initiated by surface nucleation, has great impacts on interfacial transfer of mass, heat, electrons, and light. It causes profound performance decline in numerous industrial processes, such as impaired heat transfer in heat exchangers and boilers, increased pressure drop in pipes, flow blockage in filtration membranes, corrosion damage of steam turbines, decreased conductivity and activity of electrodes, premature failure of heating and electrochemical components etc.6. Both processes are strongly influenced by material surface properties. Similar to deposition of other particles, material properties affect the attachment of mineral crystals via hydrophobic and electrostatic interactions. Surface-induced heterogeneous nucleation is a more thermodynamically favorable process, but it is poorly understood because it happens on very small time and length scales7. Few previous studies have investigated the different surface properties that influence surface-induced heterogeneous nucleation: roughness, charge, and hydrophobicity6. Surface roughness is directly related to the number of nucleation sites; it is generally recognized that mineral crystallization increases with surface roughness. However, findings on the impact of charge and surface hydrophobicity have been inconsistent. For example, some studies found that surface charge influenced the heterogeneous nucleation via electrostatic interactions or complexation reactions with the mineral ions8, while others reported similar nucleation rates on surfaces of different charges9. Contradictory results have also been reported on the role of surface hydrophobicity in mineral scaling. Hydrophilic coatings such as graphene oxide (GO), graft polymers, and polyethylene glycol, were shown to delay the onset of CaCO3 scaling in some studies12, while other studies showed that hydrophilic surfaces promoted CaCO3 nucleation8, and GO had no anti-scaling effect13. One possible reason for such apparent contradiction is that modification of a surface property  often leads to inevitable changes in other surface properties, which makes it difficult to discern the role of an individual surface property for development of anti-scaling materials.Scale formation can occur through the deposition of mineral crystals formed in the bulk solution, as well as through surface-induced heterogeneous nucleation with crystals growing from nucleation sites on a surface14 and metal oxides15 in nonaqueous phases. Theoretical and experimental research on water-surface interactions21 and related phenomena, such as ultrafast water transport26, de-icing27, and anti-fouling28, also suggest that the atomically smooth morphology plays an important role in preventing adhesion. However, no studies have investigated the scaling behavior, especially heterogeneous nucleation, on the surface of 2D nanomaterials such as graphene and hexagonal boron nitride (hBN) in aqueous solutions. Furthermore, 2D materials vary widely in surface chemistry despite their common feature of atomic-level smoothness: Graphene consists of a single layer of carbon atoms arranged in a honeycomb lattice nanostructure, featuring a small lattice constant, low in-plane polarity, and high hydrophobicity; hBN, another important 2D material with its lattice structure and lattice constant similar to those of graphene30, has high in-plane polarity due to the boron-nitrogen bond and higher hydrophilicity than graphene18. It is unknown how such differences in surface chemistry influence mineral scale formation on these atomically smooth surfaces.Two dimensional (2D) materials possess atomically smooth surfaces, and have attracted tremendous interest for their potential applications in processes where interfacial interactions play a critical role. For example, graphene was shown to effectively inhibit nucleation of metalshBN surface, and compare it to that on metal ) and polymer ) surfaces commonly used for high scaling potential applications  of 3.28) and characterized using a video camera for 220 min  is much shorter than the operation time, both bulk precipitation due to homogeneous nucleation and surface induced heterogeneous nucleation contribute to scale formation. As shown in Fig. 4 crystals were found on the pristine Cu surface after 20 min, and severe scale formation (0.123 ± 0.014 mg mm−2) was observed after 220 min of testing. On the graphene surface, CaSO4 crystals occurred after 40 min, and increased to 0.015 ± 0.008 mg mm−2 at the end of the experiment  during the 220 min experiment, except at its edges where the hBN nanocoating was damaged due to sample cutting  two orders of magnitude lower than CaSO4 (19.10 mmol L−1). It allows using much lower concentration to prepare supersaturated CaCO3 solution with saturation index similar to supersaturated CaSO4, which would generate smaller amount of scalants on the surfaces and is conducive to performing microscopic ex situ or in situ characterizations. As shown in Fig. 3 (SI of 1.18) was introduced to the test surfaces immediately after preparation before homogeneous nucleation occurred , while 55.6 ± 3.3, 13.2 ± 4.7, and 3.6 ± 0.8 vaterite crystals mm−2 were found on Ti, PVDF, and graphene surfaces, respectively  technique  led to an immediate albeit small change in frequency and dissipation, which reached equilibrium in ~1 h. The larger frequency and dissipation changes observed for the Ti and PVDF surfaces suggested more adsorption of Ca2+ and CO32− ions than that on the graphene and hBN surfaces. The similar frequency and dissipation changes on graphene and hBN surface suggested that they had similar ion adsorption, a result of their similar surface charge32. After the initial ion adsorption, greater frequency and dissipation changes occurred, and exhibited two distinct stages. The first stage (stage i) featured relatively slow changes in frequency (dF/dt) and dissipation (dD/dt), signaling the formation of vaterite nuclei, i.e., the induction process. This was followed by a much faster frequency shift (stage ii), which signaled crystal growth from the nuclei previously formed on the surface , which are atomically smooth with no observable sags, crests, or other surface irregularities. During the nucleation and crystal growth stages, the nanoscale rough features on Ti and PVDF serve as heterogeneous nucleation sites, provide cratered surface for nuclei attachment, and increases friction that may hinder local flow velocity33, all conducive to vaterite nucleation and crystal growth, resulting in a shorter induction stage and faster vaterite growth  > PVDF (27.6 ± 3.4 μN μm−2) > hBN (12.1 ± 4.3 μN μm−2) ≈ graphene (11.1 ± 6.1 μN μm−2), correlating well with the surface roughness  and moderate carbon atom electronegativity (2.55)  is therefore notably higher than B (2.04), leading to corrugated charge distribution on hBN surface with a dipole moment of 0.047 D  Fig. . In hBN,7 D Fig. . The highBN surface has an important influence on its interaction with polar water molecules. The alternating negative and positive charge regions on hBN attract H and O atoms in water, respectively, leading to stronger interaction with water molecules than on graphene surface, where the charge is more uniform. As a result, water molecules are more closely packed and located closer to the hBN surface: the average distance between water molecules is 2.45 Å, and the closest distance between water molecule and the hBN surface is around 2.92 Å  is required for a Ca2+ ion to penetrate the hydration layer to reach the hBN surface than the graphene surface  is smaller on hBN than on graphene  are smaller than those reported for conventional material surfaces, such as NaCl(001) (~3.0 Å51), Cu(110) (2.8 ± 0.2 Å52), and Au(111) (2.71–2.88 Å53), as well as the calculated intermolecular distance between water molecules in bulk water (2.81 Å). The denser interfacial water layer formed on the 2D ultra-smooth graphene and hBN surfaces may be related to their small atom size and lattice constant that closely matches the size of water molecule. Most conventional materials have lattice constants larger than 3.0 Å (Supplementary Table hBN (2.50 Å) match the water molecule size better, with hBN’s lattice constant being the closest to a water molecule size. The close match between the distribution of hBN surface energy corrugation (charge nonuniformity) and the di-pole of water molecules leads to strong polar interactions between the hBN surface and polar water molecules, and hence compact packing of water molecules on hBN surface. Combining an atomically smooth surface, large in-plane polarity, and a lattice constant matching best with the size of water molecules  pipes using the chemical vapor deposition (CVD) method . These coatings can protect substrate materials from scaling or corrosion62, as well as ensure energetically efficient fluid flow, high heat transfer rate, and long material lifetime.Overall, the unprecedented scaling resistance of hBN possibly the most anti-scaling material known, with notable advantages over graphene. The atomic smoothness of both hBN and graphene surfaces reduces the number of surface heterogeneous nucleation sites as well as the binding force between the mineral crystal and the surface, resulting in much greater resistance to mineral scaling compared to conventional scale-resistant materials. Interestingly, hBN exhibits even greater resistance to surface-induced heterogeneous nucleation than graphene, which stems from the in-plane dipole of the boron-nitrogen bond at a scale closely matching that of water molecules, leading to in-plane atomic energy corrugation that favors interaction with water. Direct experimental measurements and DFT calculations show that such chemical structure results in the formation of a dense hydration layer on the hBN surface, which strongly hinders mineral ions from approaching the surface and hence suppresses the heterogenous nucleation process. The hBN nanocoating grown on a stainless-steel tube exhibits outstanding anti-scaling properties in real oil & gas produced water, demonstrating its potential application in practical engineering systems. The results of the study provide important insights for future development of novel functional materials by manipulating their interactions with surrounding media. On the other hand, the scalability, durability, and long-term scaling resistance of the hBN coating as well as the specific roles of substrate materials and defects in large scale coatings need to be evaluated before practical applications can be possible.In summary, we discover that the unique combination of atomically smooth surface, high in-plane polarity, and proper interatomic spacing (lattice constant) makes hBN coatings were formed on Cu foil . Before coating, the Cu substrate was electrochemically polished. Ti films of 40 nm in thickness were formed on the Cu substrate using a sputter coater  at a current of 30 mA. PVDF was coated on the Cu substrate via spin coating. A 1wt % PVDF in DMF solution was applied at a rotation speed of 2000 rpm for 1 min, followed by overnight drying at 60 °C before use.Ti, PVDF, graphene and hBN films were formed on Cu substrate by chemical vapor deposition (CVD). During growth, the electrochemically polished Cu foil was first loaded into a tube furnace and heated to 1000 °C, followed by annealing at 1 Torr 15% H2/Ar for 20 min. The graphene growth was then carried out by feeding 15% H2/Ar (flow rate of 100 sccm) and methane (flow rate of 10 sccm) to the furnace for 20 min. For hBN growth, ammonia borane was used as the precursor. After Cu foil annealing, ammonia borane was heated to ~85 °C for evaporation upstream of the tube furnace, and was carried by H2 gas to the Cu substrate to grow hBN at 1000 °C for 30 min. After growth with graphene or hBN, the Cu foil was rapidly cooled down to room temperature for further use.Graphene and hBN were characterized by Raman spectroscopy  and X-ray photoelectron spectroscopy (XPS)   and atomic force microscopy (AFM) . The chemical property of the graphene and hBN nanocoatings were formed on flat Cu substrates, and their scaling resistance was evaluated in a solution supersaturated with CaSO4, a common scalant in ground water that was very difficult to remove. The supersaturated CaSO4 solution (50 mmol L−1) was prepared by mixing 100 mmol L−1 CaCl2 with 100 mmol L−1 Na2SO4 at a 1:1 volume ratio. The prepared solution had a pH of 7.0 and a saturation index (SI) of 3.28. SI is defined as the ratio between the chemical activity product of the mineral ions and their solubility product63. During the experiments, the pristine Cu foil, hBN- and graphene- grown Cu foils were immersed in the CaSO4 solution. Scale formation was monitored using a video camera for 220 min : Ca2+ and CO32− activity are 1.23 × 10−4 and 1.24 × 10−4 mol L−1, respectively; pH is 8.84, ionic strength is 5.56 × 10−3, and calcite, aragonite, and vaterite saturation indexes ({Ca2+}{CO32−}/Ksp) are 4.45, 3.18, and 1.18 respectively. The induction time for homogeneous nucleation was determined by dynamic light scattering (DLS) measurements using a NanoBrook Omni    technique , which has a mass sensitivity and a dissipation sensitivity of 1.8 ng cm−2 and 0.1 × 10−6 in liquid, respectively64. To perform the QCMD experiments, the four test materials were first coated on QCMD sensors. Ti and PVDF were coated on QCMD sensors using the same methods as those used for coating the Cu substrate. Graphene and hBN were first grown on Cu substrate, and then transferred onto QCMD sensors using a PMMA-assisted method. In this method, a PMMA film was spin-coated on the graphene- or hBN- grown on the Cu foil. The sample was then immersed in a FeCl3 solution to dissolve the Cu substrate. The graphene or hBN layer immobilized on the PMMA film was then transferred to DI water, and subsequently collected onto the QCMD sensor surface with the graphene or hBN side attaching to the sensor surface. Finally, acetone and IPA were used to dissolve the PMMA layer, exposing the graphene or hBN surface. The coated sensors were then mounted in 4 parallel QCMD cells, and characterized for CaCO3 nucleation and growth at a temperature of 22 °C using the same supersaturated solution and flow rates as the offline scaling experiments described above. The frequency and dissipation data were continuously monitored for each coated sensor.The heterogeneous nucleation and crystal formation of CaCO−2) on the sensors was calculated from the frequency change using the Sauerbrey equation:−2 Hz−1 in this study. Δf is the frequency change (Hz). n is the harmonic number.The increase of adsorbed or precipitated mass  the interaction between water molecules and hBN or graphene surface, (iii) the interaction of Ca2+, CO32−, and CaCO3 with bare hBN and graphene, and (iv) the interaction between Ca2+, CO32−, and CaCO3 and hydrated hBN or graphene. Detailed calculation and simulation methods can be found in the Supplementary Note In this study, we used density function theory (DFT) to evaluate (i) the charge distribution on graphene and hBN surfaces utilizing N2/H2O mixed gas at different humidity levels. In these experiments, a customized humidity control device was installed between a N2 gas tank and the QCMD analyzer  concentration of 108 mg L−1 and conductivity of 148 mS cm−1. Detailed water quality data are shown in Supplementary Table 2 or Na2SO4 to prepare the respective Ca2+- or SO42−- rich solutions. During the experiment, the two solutions were continuously fed into an inline mixer at 0.1 mL min−1 and then flowed through the pristine and hBN coated SS tubes at a combined flow rate of 0.2 mL min−1. Scale formation was monitored using a camera during 14 days of operation.We also grew a Supplementary InformationPeer Review FileSupplementary Movie 1Supplementary Movie 2Supplementary Movie 3


We prepared Ca-doped yttrium oxyhydride thinfilms ((CazYz1–)HxOy) by reactive magnetron cosputtering with Ca doping concentrationsbetween 0 and 36 at. %. All of the films are semiconductors with aconstant optical band gap for Ca content below 15%, while the bandgap expands for compositions above 15%. Ca doping affects the photochromicproperties, resulting in (1) a lower photochromic contrast, likelydue to a lower H– concentration, and (2) a fasterbleaching speed, caused by a higher pre-exponential factor. Overall,these results point to the importance of the H– concentrationfor the formation of a “darkened” phase and the localrearrangement of these H– for the kinetics of theprocess.To develop an understanding of the photochromic effectin rare-earthmetal oxyhydride thin films (REH Ix3–2Ox) are multianion compounds whichhave gained attention in recent years because they exhibit a photochromiceffect.1 Thin films of REHx3–2Ox 6 are transparent semiconductors which “darken”, orbecome opaque, upon exposure to light with photon energy greater thantheir band gap (Eincident > Eg). When this light is removed, the materialsreturn totheir transparent state. Although this so-called photochromic effectis promising for applications such as smart windows, the precise mechanisminvolved in this effect is yet unknown.Rare-earth metal oxyhydrides oxyhydrides16 and  oxychlorides18 resulted in improved anion mobility.Here, we dope yttrium oxyhydrides with calcium (Ca2+ vsY3+) to assess the effect of this on especially the kineticsof the photochromic effect. Until now, it has been shown that a largerO:H ratio results in a faster bleaching speed, but it is not clearif this is due to the increase in O2– content orthe associated anion vacancies.4What all these ideas have in common is the displacement ofH– ions appears to be reduced,while the concentration of O2– increases slightly.Above a Ca content of ∼15%, the lattice is strained anisotropically,and the optical band gap expands, which may be related processes.All of the films are photochromic and show a reduction of the photochromiccontrast with the substitution of Y for Ca. We propose that the Casubstitution reduces the fraction of octahedral H– and that these entities are important for formation of a “darkened”phase. The bleaching speed is faster as Ca is substituted into thestructure due to an increased pre-exponential factor which we attributeto the greater fraction of octahedral vacancies.We show that we can successfully dope yttrium oxyhydride thin filmswith calcium in the range 0–36%. To compensate for this substitution,the concentration of HIIzYz1–)HxOy, ∼ 300 nm) were preparedby reactive magnetron cosputtering of Ca  and Y  metal targets onto 10 × 10 mm2 quartz plates(MaTecK) at room temperature (∼21 °C) and an Ar/H2 (7:1 flow) atmosphere. Following from our previous work onREHx3–2Ox thin films ,5 the combined Ar/H2 deposition pressure (pdep) affects the as-deposited RE dihydride; we found thatif pdep is below a critical pressure (p* ∼ 0.4 Pa for Y),4 the film remainsa metallic RE dihydride, but above p*, it forms asemiconducting oxyhydride upon ambient air exposure (2O] and [O2] < 0.1 ppm), perhaps due to oxidation fromresidual gases.19 For this work, we usedonly one pdep of 0.5 Pa to survey theeffect of Ca doping on the photochromic properties using a range ofCa concentrations (at. %). Co-sputtering was achieved by alteringthe input DC power to the two targets while keeping a constant totalmetal flux  of YH2 and Ca . Before deposition, the chamber was kept at a base pressurebelow 10–6 Pa.Ca-doped yttriumoxyhydride thin films ((Caexposure 1. SimilazYz1–)HxOy films was measured by ion beamanalysis using Rutherford backscattering(RBS) and elastic recoil detection (ERD). RBS (ERD) was performedat the DIFFER Ion Beam Facility using a 2.5 MeV 4He ionbeam at a 0° (75°) incident angle and 170° (23°)scattering angle. The former is used to analyze heavy elements , while the latter is sensitive to light elements (H). For ionbeam analysis, the (CazYz1–)HxOy thin films were deposited onto glassy carbon substrates (8× 8 mm2) and Si wafers with a native oxide . The thickness of thesefilms was ∼150 nm. RBS/ERD data were fitted by using SIMNRA.21The composition of the (Ca+) implantation energy between 0.1 and 25keV with the variable energy positron beam (VEP) facility at the ReactorInstitute Delft. The energy distribution of the annihilation γ-rayswas measured with a high-purity Ge detector (cooled by liquid nitrogen)which has an energy resolution of 1.2 keV. The resulting S and W parameters were fitted by using the VEPFITprogram.Dopper broadening positron annihilation spectroscopy (DB-PAS)wasused to probe the phase nature of the Ca-doped yttrium oxyhydridethin films. Depth profiles were collected at room temperature by varyingthe positron  was usedto study theeffect of Ca doping on the crystal structure of Y-oxyhydride thinfilms in grazing incident geometry  and a Cu source. To find the Ab-initio Simulation Package(VASP)23 on model structures of (CazYz1–)Hx–z3–2Ox based on the special quasi-random structures (sQS) of our previouswork.24 Within the scheme of the projectoraugmented wave (PAW) method,26 a plane-wave basisset is used and periodic boundary conditions are applied. Standardfrozen core PAW potentials are used, and the H 1s, O 2s2p, Y 4s4p4d5s,and Ca 3s3p4s are treated as valence shells. For each structure, allcell parameters and atomic position are simultaneously optimized employingthe PBE generalized gradient approximation for the exchange-correlationfunctional.28 After that, the modified Becke–Johnson(mBJ) exchange potentials in combination with L(S)DA-correlation havebeen used to compute the electronic properties.30 In all cases, integrations over the Brillouin zone are performedon a 3 × 3 × 3 Γ-centered K-mesh by using a Gaussiansmearing of 0.05 eV, and convergence (δE <0.1 meV) is reached with a kinetic energy cutoff of 850 eV.First-principles density functionaltheory (DFT) calculations wereconducted with the Vienna 31 . Photochromism was measuredby illuminating the thin films for 1 h with a narrow wavelength LED. The average transmission (λ = 450–1000 nm) was plottedwith respect to time at room temperature (∼21.5 °C). Afterillumination, the film was left to “bleach” until itsoriginal transparency was recovered. Temperature sweeps were donewith the addition of heating at the sample stage (25–55 °C).Optical transmission spectra were measured by a custom-built setupconsisting of an optical fiber spectrometer, a deuterium/quartz tungstenhalogen lamp , and a Si array wavelength-dispersivespectrometer . Optical band gap energieswere determined by using the Tauc methodIIIAx3–2Ox  describes these materials well.3 Starting from the REH1.9, upon exposure to air, tetrahedralH– is partly replaced with O2–, displacing part of the hydride ions to the octahedral positions.32 In the case of aliovalent doping of YHx3–2Ox with Ca, we expectthat one anionic charge should be removed for every Ca cation substituted.Therefore, we evaluated the compositions of our films in terms of(1) the Ca:Y ratios and (2) the relative change in the anion  content.The compositionsof rare-earth metal oxyhydride thin films (made by postoxidation ofthe as-deposited RE dihydride) have been assessed in our previouswork using RBS and ERD, finding that the empirical formula REHFigure S4) for YH1.9+δ , YHx3–2Ox ,and a series of doped Y-oxyhydrides with progressively higher Ca content.The samples are termed CaY-#, with CaY-0 having 0% Ca doping and CaY-6having the highest Ca content. Comparing first the cations, For the anions, we qualitatively conclude thatthe concentrationof O does not strongly depend on the Ca/Y ratio 2d, whilezYz1–Hx films in the gloveboxbefore air exposure  touch the transmission of the substrate, meaning thatthe films have the maximum transparency possible. Had there been ametallic phase, this value would also be lowered. As well, positronannihilation spectroscopy  canbe used to rule out the presence of small metallic secondary phasesbecause the positron may annihilate preferentially in metallic centers.36 When a significant amount ofCa metallic domains would have been formed, a larger increase in thepositron Doppler broadening S parameter is expectedthan what is observed here.37 Two samples with either0 or 20% Ca have nearly the same S parameter , suggesting that no metallic phases are present in either caseand that their cation vacancy structure is very similar.The phase nature of these films is important to assess becauseCa is a highly reactive element with a strong tendency toward oxidation,and one can imagine that phase segregation may occur where Ca createsa secondary phase within the Y-oxyhydride matrix instead of participatingin aliovalent doping of the oxyhydride. First, we address the presenceof a metallic Ca phase within the oxyhydride. From optical transmissionmeasurements of the Caexposure 1a, the a2 forms a secondaryphase within the Y-oxyhydride matrix. However, from the optical transmissionmeasurements shown in 2 phase (∼4.4–5.2 eV).39 This is likely due to the partial pressure of H2 usedduring sputtering, which may be too low to achieve the CaH2 state . The appearance ofan optical band gap before air exposure could instead be due to smallamounts of O2 contamination in the glovebox introducedduring sample transfer. As well, the reduction of the H peak fromERD with the addition of Ca suggests that there is no CaH2 formation  withinthe Y-oxyhydride matrix. On the basis of the opticaltransmission spectra of the films after air exposure exposure 1b, the texposure 3a. Thus,zYz1–)HxOy), where Ca substitutes for Y in the lattice (0–36%Ca), and the H– fraction decreases for charge neutrality.There are many examples of single-phase compounds containing bothyttrium and calcium such as hydrides,40 fluorides,41 and others.43 The successof this substitution involving Ca and Y may be attributed to the similarionic radii of these two elements .44 To this long list of single-phase Ca/Y compounds, we suggestto add Ca-doped Y-oxyhydrides for which we find that a stable thinfilm can be synthesized for a Ca content of at least 0–36%.Therefore, we propose that the thin films discussedhere are single-phaseCa-doped Y-oxyhydrides .45 The GI-XRD patternsin x3–2Ox with 0% Ca) and illustrate howCa doping influences these peak positions. With increasing Ca content,the (111) reflections remain at the same position as the 0% sample,while the other reflections shift to different 2θ for Ca concentrations>5%.We investigate the crystalstructures of our films using grazing incident X-ray diffraction (GI-XRD),as shown in d spacing (with respect to the undoped film) for each reflectionisshown in d111 plane is constant for all Cadoping concentrations, while the other planes expand upon increasingCa doping. Importantly, they do not expand to the same extent, withthe d200 plane expanding by 0.4% whilethe d220 and d311 planes by ∼0.6–0.7% when comparing 0% and 36% Ca.This suggests that the lattice is strained in specific directionsby the addition of Ca. This strain deforms the lattice by <1%,such that it is no longer cubic, but rather an orthorhombic latticewhere a ≠ b ≠ c. This lattice strain may be caused by the slight differencein the ionic radii of Ca2+ and Y3+ or the accumulationof vacancy defects.To better visualize this effect, the relative CThe optical bandgap energies for the films studied here are presented in 4 Here,we do not see a sudden change in the O2– contentfor CaY-4 from RBS , there is an indication that the H valence bandmaximum recedes slightly, widening the band gap by about 6%, whichis of similar magnitude as observed experimentally. It should be notedthat the cationic substitution itself does not seem to affect theband gap because the Ca states are outside the gap. Only changes inthe H– composition appear important here.In previous studies, changes in the bandgap were attributed to the composition. For example, a higher O:Hratio generally results in a wider band gap.from RBS 2d, but i46 and may play a role here.In addition, lattice strain, as observed in DT (%) = (T – T0)/T0) instead of the average opticaltransmission to normalize for slight differences in absolute transmissionand show the change in photochromic contrast more clearly. Beforeillumination, samples are transparent (ΔT =0). This increases as the samples “darken” under illumination(yellow shaded area) and decreases back to the transparent state afterillumination.The photochromic propertiesof the films are measured by illuminating them with a 385 nm LED for1 h and measuring the average transmission (λ = 450–1000nm) as a function of time. The two main figures of merit for photochromicmaterials are the contrast (maximum amount of change in transparency)and the bleaching speed . The data presented in –, making the decrease in the population of certain H– potentially significant for the observed properties of the material.Not only are the octahedral H– likely the firstto leave the structure upon oxidation,32 but they areoften cited as more mobile than tetrahedral H– eitherdue to the lower formation energy for an octahedral H vacancy14 or their weaker electrostatic interactions withO2– (more distance).47 Having these mobile H– could be an essential ingredientto the formation and dissolution of a “darkened” phase.The photochromic contrast linearly decreases withthe substitutionof Y for Ca 4b, suggeB) is derived from first-order kineticsand is related to the concentration of the “dark” species(c(t)):45B can be extractedfrom the slope of the linear fit of a ln(−ln(T/T0)) versus t plot. When the temperatureis constant, it is clear from B) versus (1/Temp) plot is related to the activation energy(EA), and the y interceptis related to the pre-exponential factor (ln(1/τ0) = ln(k0)). This is displayed in EA values for the two samples are equivalent, while the pre-exponentialfactors are different. Specifically, the 20% Ca sample, whose τB at room temperature is ∼79% lower than the 0% Ca sample,shows a pre-exponential factor that is higher by a factor of 5. Therefore,we conclude that the determining factor here for the bleaching speedis the attempt frequency.The bleaching speed, on the other hand, does notshow a linearrelationship to the amount of Ca in the samples, although a monotonousdecrease can be recognized 4b. To unB is related to the conventional diffusionof H– from the “darkened” phase toits original position, the attempt frequency could be explained bythe amount of vacant sites, hopping distance, and other factors. Givenour previous reasoning on the relation between the Ca doping and theamount of octahedral hydrogen, an explanation involving the increasingamount of octahedral vacancies is the most consistent one and pointsto a short-range diffusion mechanism related to bleaching.On the other hand, for RE oxyhydrides with proven long-range H– conductivity (RE = La),48 changes in this conductivity were also attributed to thepre-exponential factor rather than the EA, but citing complex interactions of many H– ionsas the source.The rationalization of the attempt frequency in thiscontext isnot straightforward because this parameter can be interpreted in manyways. If τIV– that may be essential for the creationof a “darkened” phase. The bleaching speed became fasterdue to Ca doping, indicating a potential relation between the bleachingspeed, the attempt frequency, and the number of octahedral vacancies.These results point to the importance of local H– diffusion for the understanding of the photochromic mechanism, althougha full explanation of this effect should account for other aspectsof these materials such as anion disorder and other inhomogeneities.In conclusion, we haveprepared single-phase aliovalently dopedyttrium oxyhydride thin films with Ca (0–36%). These filmswere made by reactive magnetron cosputtering and air oxidation toachieve the oxyhydride phase. The composition of the cations was verifiedby RBS, and qualitative analysis of the anions (by RBS and ERD) showedthat the O content was largely unaffected by Ca doping, while theH content decreased. As well, the addition of >15% Ca resultedinthe appearance of anisotropic lattice strain and a moderate expansionof the optical band gap, two effects which may be related. Importantly,all of these films are photochromic, showing that the photochromiccontrast decreases with the addition of Ca, possibly due to the removalof octahedral H


The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ18O values are invariable  and indistinguishable from “normal” upper mantle, in contrast to significantly lower δ18O values reported for erupted materials elsewhere in Iceland . Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for 18O-depleted mantle or interaction with low-δ18O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system.The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ The 2021 eruption in the Reykjanes Peninsula of Iceland was the first in 800 years and was supplied by melts from diverse mantle source domains with near-identical oxygen isotope ratios, providing a unique insight into the Icelandic mantle plume. Until March 19th 2021, when a low-intensity effusive basaltic eruption began at Fagradalsfjall  in Western Iceland is not only a volcanically active plate boundary functioning as the onshore extension of the Reykjanes Ridge, but it also hosts about 70% of Iceland’s population, including the Greater Reykjavík area, the Svartsengi and Reykjanes geothermal power plants, and Keflavík international airport. The RP is a transform zone where highly oblique spreading at a rate of 18.8 mm/year gradually pushes the North American and Eurasian plates apartula Fig. . In the 4. In total, 39,500 earthquakes, as well as a period of strong inflation, were detected in 2020 in a region just 8 km to the west of the Fagradalsfjall eruption site5. This seismogenic zone is underlain by a brittle-ductile transition at 6–7 km depth6 and the Moho at around 15 to 20 km depth5. In light of the volcanic history and the geodynamic setting of the RP, the 2021 Fagradalsfjall eruption most likely represents an onset of a new eruption period, which may last centuries. Surface deformation and crustal seismicity on the RP can be explained by geothermal fluid mobilisation and ascent into shallow sealed zones along with instability of a magma reservoir in the lower crust or upper mantle between 15 and 20 km depth5. Therefore, it is important to understand the processes and composition of the mantle sources that fed the Fagradalsfjall eruption , to better assess magma and hydrothermal fluid production in this area from a geological, volcanological and economic point of view.The RP has also been the location of large shallow tectonic earthquakes with magnitudes up to M6 that are typically confined to the upper crust to a depth of ca. 6 km10). Stable isotopes of oxygen  are a particularly robust tracer of crustal and mantle inputs to Icelandic magmatism because (i) oxygen is a major (ca. 50 wt%) component of magmatic rocks, (ii) oxygen isotope ratios are only minimally fractionated at the high temperatures characteristic of mantle processes, but undergo large fractionations at low-temperature surface conditions, and (iii) mantle and crustal components have distinctly different oxygen isotope ratios , making it possible to effectively trace crustal inputs to mantle-derived melts. Oxygen isotope studies on Icelandic rocks and minerals have shown that many Icelandic basalts, including their crystals and melt inclusions, display a large degree of oxygen isotope diversity spanning several permille from around +2 to +6‰ . This range is far beyond the variation that is commonly ascribed to magmatic processes and significantly different to “normal” ambient upper mantle, which has relatively uniform δ18O values of ca. 5.5‰ . The reasons for this large degree of oxygen isotope diversity in Iceland remain uncertain and the question of whether the observed diversity is the result of an original mantle source with heterogeneous δ18O values or crustal assimilation involving hydrothermally altered, low-δ18O basaltic crust, or a combination of the two, remains unresolved (cf. ref. 23).Despite decades of research, there is a long-standing debate concerning the Icelandic geodynamic setting and the exact nature of the plume components involved in magma genesis. Geochemically enriched as well as depleted components  have been suggested to be delivered to the sub-Icelandic magmatic system by a deep plume source under Iceland , stable isotopes of hydrogen , and water contents (H2O in wt%) to further constrain Fagradalsfjall magma source characteristics. In brief, the Fagradalsfjall samples analysed here display variations in their incompatible trace element ratios that can be linked to distinct mantle components. As a consequence of these trace element variations, we now have the unique opportunity to test for potential coupling of δ18O values to various mantle components and to assess hypotheses relating to the oxygen isotope composition of the mantle underlying the Reykjanes Peninsula.This paper contributes to the Icelandic oxygen isotope debate via geochemical characterisation of pristine and relatively primitive eruption products from the 2021 Fagradalsfjall eruption, where we have identified involvement of at least three different mantle source components in magma genesis. We present major and trace element data as well as δ24). On March 19th 2021 a low-intensity effusive basaltic eruption, the first on the Reykjanes Peninsula for 781 years , commenced via several small vents along a 180 m long, north-northeast-trending fracture set in Geldingadalir within the Fagradalsfjall volcanic complex . The main period of sampling for our study was characterised by magma discharge of 6 ± 0.5 m3/s (period 19th March–13th April) and 9.8 ± 0.7 m3/s (period 13th April–2nd July)  that are close to being in equilibrium with the erupted melt. Normally zoned plagioclase and olivine macrocrysts feature cores with fairly level An87–89 and Fo87–89 compositions, respectively. Reversely zoned plagioclase and olivine macrocrysts contain an outer zone of An87–89 in plagioclase and Fo86 in olivine and a core of An86–84 in plagioclase and Fo84–85 in olivine . The geophysical evidence therefore supports a scenario where the eruption drained a lower crustal magma storage reservoir, as is typical for Icelandic rift zones27. This realisation permits us to investigate the sources of the melts feeding this storage zone and to constrain how the composition of magmas supplying the eruption changed over time.The lava at Fagradalsfjall is olivine tholeiite and contains ca. 2–15% modal abundances of plagioclase (up to ca. 6 mm in size) and olivine (up to ca. 5 mm in size) set in a finely crystalline to glassy groundmass ; Fig. 28). In Fig. We analysed 30 lava and tephra samples for bulk-rock major and trace elements. All major element data were normalised on a volatile-free basis before plotting and the normalised values are reported here , as shown in Fig. 17O . Considering the data on a month-by-month basis, the mean δ18O value for March, April, May, June, July, and August is 5.3, 5.4, 5.3, 5.4, 5.5, and 5.3‰, respectively . Given that the natural variability on the oxygen isotope data of glass is 0.2‰ , the monthly mean δ18O values for Fagradalsfjall are indistinguishable. The δ18O values obtained here furthermore overlap within analytical uncertainty with values reported for MORB  and, importantly, Fagradalsfjall samples with diverse trace element ratios have identical δ18O values. Regarding Δ′17O, the values presented here are among the first reported for fresh Icelandic basalts and range from −0.031 to −0.048 ± 0.012‰. These values overlap the mantle range . Except for the most degassed sample (δD = −109.7‰), these values overlap within analytical uncertainty with the canonical range for pristine MORB of −80 ± 10‰ (see ref. 29) but are ca. 10‰ higher than North Atlantic depleted MORB29. We furthermore performed eight new analyses of MORB basaltic reference material , which was run concurrently with the Icelandic unknowns and yielded δD values ranging from −72.2 to −87.3‰ with a mean of −79.2 ± 10‰ . While performing the hydrogen isotope analyses, we also performed 17 water analyses on Fagradalsfjall glass samples. However, several of these samples were anhydrous, containing less than ~0.03 wt.% water, which was not detectable by the High Temperature Conversion Element Analyzer employed here  and thus these samples have no accompanying δD value. The H2O contents of our hydrous Fagradalsfjall glasses range from 0.00 to 0.23 wt.% and the UOB MORB reference glasses analysed here have H2O contents ranging from 0.38 to 0.53 wt.%. The subaerial and partly degassed Fagradalsfjall samples are thus less hydrous than the submarine MORB glass standard (UOB), which was quenched at ocean floor depths.The δD values for our Fagradalsfjall samples range from −69.2 to −109.7‰ and yield a mean of −81.8 ± 26‰  Fractional crystallisation of a single basalt parent magma; (ii) Crustal assimilation modifying a basaltic parent magma ; (iii) Different mantle melting degrees with a single source, but displaying variations in the depth/temperature of melting; and (iv) Involvement of closely spaced and simultaneously tapped multiple mantle sources/endmember compositions such as depleted or variably enriched peridotite-pyroxenite plume components as previously suggested to reside under Iceland.5), which is consistent with magma storage in a lower crustal reservoir (cf. ref. 27) as opposed to at shallower levels where protracted crystal fractionation  would be expected to occur.Hypothesis (i)  can be excluded as the principal cause for the large variation of incompatible trace elements observed here, because the studied samples do not display systematic major element variations characteristic of fractional crystallisation of olivine + plagioclase ± clinopyroxene see Fig. . Geophys32) as evidenced by hydrothermally altered rocks in deep drill holes at the tip of Reykjanes. Oxygen isotopes are sensitive to incorporation of even small amounts of hydrothermally altered, low-δ18O and low-δD crustal materials, which in Iceland can differ from mantle-derived materials by several permille (‰) . As both oxygen and hydrogen isotopes point to typical MORB-like composition for Fagradalsfjall   can also be excluded as the dominant cause of the observed temporal variations because of the essentially homogeneous oxygen isotope composition of the Fagradalsfjall lavas. The crust under Reykjanes is highly diverse with respect to its oxygen isotope composition and even more so with respect to hydrogen isotopes .Similarly, hypothesis (iii) (mantle melting variation of a single source) is unlikely, since the degree of melting would need to vary by a factor of two or more to fully explain the observed variations of incompatible trace elements by variations in mantle melting degree alone. This is because such variation in melting degree should leave ratios of similarly incompatible trace elements, such as K/Ti, Zr/Nb relatively invariant, which is not what is observed in our temporal dataset  with variable mixtures of an N-MORB envelope and a plume of E-MORB type mantle and enriched (OIB) plume centre. Murton et al. (ref. 10) offered correlation diagrams employing incompatible trace element ratios (Nb/Zr vs Zr/Y) that require mixing of at least three components: the low trace element ratio components characteristic of the Reykjanes Ridge (D1 and D2) and an enriched source (E1). As can be seen in Fig. 10). Interpreting our data in the context of, e.g., Fitton et al. and Kempton et al. (refs. 9), who suggested that the mantle beneath the southern Reykjanes Ridge is E-MORB-like, and Murton et al. (ref. 10), who suggested that the Eldgjá eruptives (E) are representative of the enriched plume, the 2021 Fagradalsfjall eruption may have been fed from what previous workers termed the Icelandic “plume sheath”. This sheath possibly envelopes a more strongly enriched plume centre and may be compositionally zoned . We also note that enriched mantle domains have been shown to retain higher melt fractions than more depleted domains as they cool36, which might in part explain the increase in the enriched component over time at Fagradalsfjall.Thus, hypothesis (iv) (melting of diverse mantle types) appears to provide the best explanation for the variations we observe in our data. In terms of their incompatible trace element ratios, Fagradalsfjall basalts are similar to previously published enriched samples from the Reykjanes Peninsula and they are distinct from depleted Reykjanes Ridge samples Fig. . Fagrada18O values obtained for Fagradalsfjall basalts are remarkably homogeneous over the eruption lifetime  and are thus considered more accurate than previous estimates for MORB of ca. 5.7 ± 0.3‰ . Older estimates relied on bulk analyses using conventional fluorination of powdered glass samples, an approach, which has now been superseded by modern techniques for analysing oxygen isotopes in silicate materials . In this respect, it is particularly noteworthy that the new Fagradalsfjall δ18O values are significantly different to values obtained from Central Iceland, some of which were obtained using the same analytical techniques as those employed here (laser fluorination). This difference is exemplified by the 2014–2015 Holuhraun eruption, which yielded mean δ18O values of 3.8‰ , making Holuhraun basalts on average 1.6‰ isotopically lighter than Fagradalsfjall basalts  more depleted than average global MORB22. Therefore, despite there being evidence for multiple distinct mantle components involved in Fagradalsfjall magma genesis, our data support the absence of 18O-depleted mantle supplying the Reykjanes Peninsula. This contrasts with studies focussed on other parts of Iceland, such as Theistareykir (Northern Volcanic Zone), where the mantle source has been discussed as having variable and low δ18O values40. We offer the suggestion that if the oxygen isotope values presented in this study are representative of various Icelandic plume components, then that would imply that the mantle underlying Iceland has a relatively uniform primary δ18O value. If this is indeed the case, then a follow-on implication might be that high-Mg basaltic lavas with low δ18O values at, e.g., Holuhraun are the result of assimilation of low δ18O crust or of delamination of low δ18O crust into the mantle source. These regional aspects are, however, speculative and somewhat beyond the scope of the current work but they may be an attractive target for future studies.The δe37 Fig. . The reclts Fig. . A key o29). The samples are also overall less hydrous than both MORB and Reykjanes Ridge samples41 , but degassing leads to a decrease in water content and δD value. If degassing from a primary magma with 0.6 wt.% H2O is incremental and of Rayleigh type, it can result in a change of up to 20–30‰ in the δD value of the residual basaltic glass43, but only a change of 10‰ would be expected if the degassing process is of Batch-type44. We modelled both types of degassing employing starting compositions similar to the UOB MORB glass standard . The first proposed idea involves discrete domains (“streaks” in the context of a streaky or plum-pudding type mantle models) of more fertile, perhaps pyroxenitic mantle, dispersed in an otherwise less enriched (E-MORB) Icelandic plume matrix , but much of this water was lost during near-surface degassing, a process which also modified the lava δD values  in a lower crustal storage reservoir at ca.15–20 km depth . Purified BrF5 was used as a reagent and a boiling mercury diffusion pump was used to remove excess F2 gas remaining after reaction of BrF5 during laser fluorination. Purified O2 was converted into CO2 and then run in dual inlet mode on a MAT253 mass spectrometer integrated with a vacuum line. San Carlos Olivine (δ18O = 5.25‰), UWG2 garnet (δ18O = 5.80‰) and an indoor UOG garnet standard calibrated relative to the other two (δ18O = 6.52‰) were used to calibrate the data on the VSMOW scale. Each session included analyses of four to six standards and correction for day-to-day variability was 0.0 to 0.2‰. Reference CO2 gas was used as a working standard and it was periodically rerun against OZTECH CO2 gas. Standard deviations on standards in individual sessions ranged from ±0.00 to ±0.11‰ . Samples from sessions with less precise standard data were rerun and the average was used for plotting purposes with the full duplicates/triplicates reported in the 2 along with the SCO standard (Δ′17O = −0.052‰) and a triple O continuous flow line specially constructed for precise triple O measurements was utilised. The generated O2 gas was run through an 8 ft long gas chromatographic column at room temperature for purification of NFx compounds. Generated gases were additionally frozen on a 5 Å zeolite sieve and then released into the bellow of the mass spectrometer by an LN2-ethanol mix over 10 min. GC and zeolite traps were degassed with flowing He at 200 °C for 15–20 min between each sample. The purified O2 gas was run five times with eight cycles in dual inlet mode against a well calibrated oxygen gas standard on a MAT253 isotope ratio mass spectrometer. For triple oxygen isotope analyses of the Icelandic samples reported in this study, four concurrently run UWG standards were used to correct data to the VSMOW scale. Additionally, a set of 26 olivine grains from mantle nodules (quoted as average) run before this session provided another point of calibration for the obtained Icelandic dataset with that of the mantle. Our indoor San Carlos Olivine has the following values: δ18O = 5.25‰ and Δ′17O0.5305 = −0.052‰.All oxygen isotopic data in this study were obtained at the University of Oregon (UO), Eugene, Oregon, USA. Detailed method descriptions can be found in refs. 47 and are summarised here. Samples weighing between 8 and 18 mg were loaded into Ag packets and dropped into a glassy carbon reactor held at 1450 °C. Water was quantitatively reduced into H2 and CO gases, which were separated by a GC column. H2 was carried over to the open split in front of the mass spectrometer by He flow, and analysis included 3 reference gas peaks and a single unknown gas peak. D/H ratios and total water in glass were determined using concurrently run standards of USGS57 and 58 micas and a MORB glass standard (UOB). Water concentration was detected by peak integration if its content was greater than ~0.02–0.04 wt.%; precision on D/H at water concentration <0.1 wt.% was ±8 ‰ and at higher concentration it was ±5‰.Hydrogen isotopes were measured on groundmass glass (50–200 µm size fraction) using a TCEA-MAT-253 system at the University of Oregon. Methods are after refs. Groundmass glasses and minerals in tephra and quenched lava samples were analysed at the University of Iceland on a JEOL JXA-8230 electron probe microanalyzer (EPMA) using an acceleration voltage of 15 kV, beam current of 10 nA and beam diameter of 10 μm. The international A99 standard was used to monitor for instrumental drift and maintain consistency between measurements.Whole-rock sample powders were analysed for major and trace elements at Activation Laboratories Ltd., Ancaster, Canada using their research grade package 4LITHORES. After fusion with lithium metaborate/tetraborate and digestion in nitric acid, major elements were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) and trace elements were analysed by inductively coupled plasma mass spectrometry (ICP-MS) on a Perkin Elmer Sciex ELAN 6000, 6100, or 9000 ICP-MS. Detection limits are 0.01 wt.% for all major element oxides, except for MnO and TiO, which have detection limits of 0.001 wt.%. Detection limits for trace elements range from 0.01 to 30 ppm. Data quality was verified by repeated analysis of internal reference materials and analytical precision is estimated at better than 5% for major elements and better than 10% for trace elements. One sample of Fagradalsfjall basalt was analysed in duplicate; the two sets of analyses are virtually identical. Information about detection limits for each element as well as the results of standard analyses and the Fagradalsfjall duplicate are provided in the Supplementary Information


These measurements were used to create the NEON Daily Isotopic Composition of Environmental Exchanges (NEON-DICEE) dataset estimating precipitation , evapotranspiration , and net ecosystem exchange  isotope ratios. Statistically downscaled precipitation datasets were generated to be consistent with the estimated covariance between isotope ratios and precipitation amounts at daily time scales. Isotope ratios in ET and NEE fluxes were estimated using a mixing-model approach with calibrated NEON tower measurements. NEON-DICEE is publicly available on HydroShare and can be reproduced or modified to fit user specific applications or include additional NEON data records as they become available. The NEON-DICEE dataset can facilitate understanding of terrestrial ecosystem processes through their incorporation into environmental investigations that require daily δ2H, δ18O, and δ13C flux data.The National Ecological Observatory Network (NEON) provides open-access measurements of stable isotope ratios in atmospheric water vapor (δ  These tracers are informative in ecohydrologic modeling applications because they provide additional points of comparison between observed and modeled environmental pools and fluxes7, and provide a benchmark with which to evaluate the performance and efficiency of modeling approaches8.The stable isotope ratios of carbon and water fluxes are natural environmental tracers that can be used to provide new insights into hydrological, ecological, and meteorological processes, as well as provide supportive metrics for understanding the complex feedbacks between the land surface and atmosphere2H/1H), oxygen (18O/16O), and carbon (13C/12C), found in water and carbon dioxide, hereafter expressed as δ2H, δ18O and δ13C values13. Water isotope ratios provide useful information for partitioning evapotranspiration (ET) into evaporation and transpiration at the ecosystem scales14, as well as understanding water use efficiency in forests, agricultural, and other ecosystems16. Carbon isotope ratios provide valuable information about the component fluxes that determine net ecosystem exchange (NEE) of carbon dioxide between ecosystems and the atmosphere17. Insights into such processes support the understanding of plant water uptake strategies and the underlying principles of water, carbon, and energy cycling in the soil-vegetation-atmosphere continuum19.Tracers commonly used in studies of land surface processes from global to local scales include the stable isotope ratios of hydrogen (https://data.neonscience.org/). These publicly available isotope datasets are part of an important network documenting hydrometeorological tracer patterns throughout North America. The NEON atmospheric gas stable-isotope measurements are collected at different tower heights, with each height being sampled approximately hourly (varying by site), enabling robust daily calculations of NEE and ET flux isotope ratios . The precipitation isotope data are collected at biweekly intervals, but they can be downscaled to a daily resolution using a validated approach22. By conducting those pre-processing steps, we can facilitate subsequent applications using these published daily flux data products.The United States National Ecological Observatory Network (NEON) collects long-term ecological data in eco-climatologically diverse field sites across the United States and provisions these data through an open-access data portal (2H and δ18O in precipitation fluxes (FP) at 25 NEON core sites and 19 NEON gradient sites, b) δ2H and δ18O of ET fluxes (FET) at 19 NEON core sites and 2 NEON gradient sites, and c) δ13C of NEE fluxes (FNEE) at 19 NEON core sites and 28 NEON gradient sites. These products form the NEON Daily Isotopic Composition of Environmental Exchanges (NEON-DICEE) datasets characterizing flux isotope ratios across diverse ecosystems over a multi-year span created using consistent instrumentation and methodology. The NEON-DICEE datasets provided here are reproducible with the published python and R scripts. The methods can be modified to fit a user’s specific application or as additional NEON data records become available in the future. The subsequent sections detail the methods used to derive isotope ratios associated with the FP, FET and FNEE fluxes, the validation of each method, and a description of the associated metadata.We generated daily records of a) δ23 using the neonUtilities R package24. A wet deposition collector at each site opens during rain events to collect samples. These precipitation event samples were combined to create a 2-week water composite which are then retrieved, filtered, and analyzed25. All datasets with stable water isotopes in precipitation (as of February 2022) were downloaded, as well as the corresponding precipitation amount data. There were 44 sites across the NEON network with water isotope data, 38 of which contained sufficient data to perform the downscaling methodology (refer to the NEON data product number DP1.00006.001 for precipitation amounts and DP1.00038.001 for precipitation stable isotope ratios). At the time of this publication, NEON’s “primary” precipitation data had a documented issue where the weighing gauge had recorded spurious small precipitation amounts25. Consequently, we used the “secondary” collector data (from a tipping bucket rain gauge) when available, and the primary data was only used at sites that did not have secondary data. As a further quality check, we removed trace precipitation events, defined here as less than 0.25 mm of accumulated precipitation in one day. This threshold can be changed within the code to generate new data products.We acquired 30-minute precipitation amount data and stable water isotope ratios in precipitation collected in biweekly intervals from the NEON Data Portal26) but remains coarser than what is ideal for land modelling applications. Consequently, by downscaling from stable isotope analyses on the bi-weekly precipitation composites to daily resolution a fine-scale product is available for future studies. The daily stable water isotopes associated with the FP were generated at each NEON site with sufficient observation data to conduct a statistical downscaling method22, which translated the observed biweekly water isotope samples to a daily estimate correlating with known precipitation amounts. The downscaling method was run 100 times to create an ensemble of FP timeseries with associated isotope ratios (δ2H and δ18O) to characterize the range of expected values. We provide all 100 ensemble realizations, as well as ensemble summary statistics (mean and standard deviation). The statistical downscaling method was described in detail and validated by Finkenbiner et al.22, here we provide a summary of the method applied to the NEON datasets.The NEON precipitation samples for isotope analysis are collected at a biweekly resolution. This temporal resolution is higher relative to past network collections  contains data mostly at monthly resolutions2H and δ18O time series was characterized using a combination of sinusoidal functions through Fourier decomposition, following the methods from Allen et al.27, and removed. The remaining isotope values were assumed to be drawn from a purely stochastic process with a mean of zero. The daily covariance statistics were predicted based on trends in the means, standard deviations, and Pearson correlation coefficients as each time series was aggregated from biweekly to coarser resolutions. Some NEON sites had few biweekly observations and were unable to be aggregated to the 12-week resolution required for the downscaling method. We anticipate as NEON continues to collect observations and the site data represents longer seasonal time scales, this downscaling method can be applied at additional sites. δ2H and δ18O stable water isotopes are strongly correlated with each other28 and often share a weaker, but significant, relationship with precipitation amount29. A Gaussian copula30 conditioned on daily precipitation amounts was used to generate pseudo-random values from the predicted de-seasonalized daily statistics of δ2H and δ18O from the stochastic signal. A copula is a multivariate cumulative distribution function used to model the dependence between random variables30, here the random variables were precipitation amount and its isotopic ratios. Lastly, the pre-defined seasonal component was added to each stochastic isotope series and a residual correction was performed. The residual correction adjusts the downscaled daily δ2H and δ18O values by forcing the biweekly precipitation weighted means of the downscaled data to match those of the observed biweekly dataset. At each NEON site with sufficient observation data, we generated 100 final synthetic daily time series which corresponded with daily precipitation amounts, seasonal signals, and stochastic variability.At each NEON site, the 30-minute precipitation amounts were aggregated to daily and biweekly totals to correspond with the biweekly tracer observations and provide a daily time series on which to condition the generated daily tracer values. The seasonal component of each δ13C-CO2, δ18O-H2Ovapor, and δ2H-H2Ovapor at established eddy covariance towers. The NEON eddy covariance bundled product31 (refer to NEON data product number DP4.00200.001) provides δ2H, δ18O, and δ13C values of the atmospheric gases at different tower heights with a typical averaging interval of 9-minutes . Gases are sampled at each height for ten minutes, with the first minute being discarded, and then cycled to the next height; thus, the sampling interval depends on the number of heights sampled per tower. These measurements were used to estimate the stable water and carbon isotope ratios associated with the daily FET and FNEE fluxes.The NEON atmospheric isotope measurements provide continental-scale ongoing measurements of δFET and FNEE fluxes requires calibration of NEON’s measured carbon and water isotope ratios to known field standards that are themselves calibrated to the Vienna Pee Dee Belemnite (VPDB) and Vienna Standard Mean Ocean (VSMOW) scales. The measured carbon and water isotope ratios frequently diverge from the VPDB and VSMOW scales due to the instrumentation drift, poor instrument calibration, or bias that has not yet been removed from the calibration routines. Fiorella et al.13 provided a calibration strategy to correct the measured δ13C values to the VPDB scale as part of the NEONiso R package24. Since that publication, NEONiso has been expanded to include calibrations for δ2H and δ18O values. We estimated the isotopic composition of the surface fluxes relative to the calibrated atmosphere fluxes by employing the Miller-Tans21 mixing model approach, which regresses the product of H2O or CO2 mixing ratios and the isotope composition of each atmospheric gases at each of the measurement heights against the respective mixing ratio for that gas32. The slope of the simple regression from this procedure estimated the isotope composition of the water and carbon fluxes. To estimate the daily isotopic value of each flux, we used the 9-minute calibrated isotope datasets across all measurement heights as the predictor of a Miller-Tans mixing model and the response was the product of the 9-minute atmospheric gas concentrations and the calibrated isotope data. We used isotope ratios of the atmospheric gases to create flux data for three time-windows per day: all day, daytime, and nighttime. For daytime data, we only used isotope ratios which occurred during time periods where the incoming shortwave radiation was ≥10 Wm−2. In contrast, for nighttime data we only used isotope ratios during periods where the incoming shortwave radiation was <10 Wm−2. The 10 Wm−2 is based on the standard threshold used in common eddy covariance pre-processing software33 where it is assumed periods with ≥10 Wm−2 of shortwave radiation only occurred during the daytime. This value can be adjusted using the provided scripts.The estimation of the daily carbon and water vapor isotope composition of the et al.34, Wehr and Saleska35). Laser-based water vapor isotope analysers have known low-vapor measurement deviations37 below a dry mole fraction of ~5000 ppm (and major below ~2000ppm). At the time of this publication, the water vapor isotope measurements provided by NEON were not corrected for low humidity deviations and the data required to correct for low humidity was not available across the network. Fortunately, most affected periods were during the winter at cold sites, where ET is expected to be low, but future work could apply these corrections to the data.While we achieved reasonable results with the Miller-Tans approach, other mass balance mixing methods can be explored in future work to estimate the isotope composition of the water and carbon fluxes  and supported by the U.S. National Science Foundation, is a web-based hydrologic information system that allows users to publish and share data repositories in a citable manner.NEON-DICEE associated data, code and metadata are available in a public repository on HydroShareFP, there are four data files containing time series of modeled δ2H and δ18O values corresponding to observed precipitation. Two are the mean of modeled δ2H and δ18O values from the ensemble output and the other two data files are the standard deviations of the modeled δ2H and δ18O. There is also a metadata file describing the quality of each data point. Each row in these files represents one calendar date and the columns are labeled with the NEON site abbreviation codes. Each column contains a daily time series of water isotope values where applicable. Days with no precipitation contain values of −9999. The precipitation flux metadata file contains quality flags for each value in a site’s time series. A “0” indicates daily isotope value was associated with observed precipitation that was larger than the defined value for trace events (here 0.25 mm), a “1” indicates there was no precipitation data on that day, and a “2” indicates the observed precipitation was less than the defined value for trace events.For the daily The precipitation downscaling techniques were successfully applied to sites with low ) and high ) annual precipitation Fig. . Across 2H, δ18O) of the FET and carbon isotope ratios (δ13C) of the FNEE are contained in three data files with row names consisting of the calendar date and column names corresponding to the four-letter abbreviation of each NEON site location (same as the FP file format described above). The associated ‘error’ files contain the uncertainty of the estimated flux. This uncertainty is the standard error of the slope coefficient in the simple linear regression of the Miller-Tans mixing model. The included FET and FNEE metadata files contain data quality flags for each flux measurement. For each stable isotope time series, a value of −9999 was used to fill days when no calibrated isotope ratios were available, and this was marked with a “1” quality flag in the metadata file. A flag value of “2” was used if there was a low number (n ≤ 5) of calibrated isotope values used to generate the daily flux values. A quality flag value of “3” was assigned when the Miller-Tans mixing model r-squared (R2) was below 0.9. To identify cases where the interred point falls outside the range of observations, an interquartile (IQR) range flag was applied to each time series meeting the above criteria ; if the inferred isotope value of the flux was beyond the 1.5 times IQR of the 25th percentile (Q1-1.5 IQR) and 75th percentile (Q3 + 1.5 IQR) of the observation data, then a flag of “4” was assigned to those data points. A “0” quality flag was used to indicate a good isotope value of the flux estimated from the isotope ratios of the gases where n ≥ 5 from the regression analysis, R2 ≥ 0.9, and the isotope value was within the desired interquartile. Users of these data products are encouraged to use the isotope values in the time series with a quality flag value of “0”. The isotope ratios of the ET and NEE fluxes which passed all quality flags at ONAQ and WREF were shown in Fig. Per time-window , water isotope ratios  ‰ to −7.8 (1.2) ‰ for δ18OET, and −29.8 (1.9) ‰ to −17.6 (1.1) ‰ for δ13CNEE  ‰ to −7.7 (1.3) ‰ for δ18OET, and −32.8 (7.3) ‰ to −18.8 (1.2) ‰ for δ13CNEE. The site averaged (±standard deviation) nighttime isotope flux values ranged from −262.4 (75.7) ‰ to −50.5 (9.5) for δ2HET, −34.3 (9.8) ‰ to −7.5 (1.2) ‰ for δ18OET, and −30.1 (1.8) ‰ to −17.2 (1.3) ‰ for δ13CNEE. Across all NEON sites, the number of δ18OET and δ2HET flux values that passed all quality flags ranged from 127 to 829 days  and 197 to 1046 days , respectively. The number of δ13CNEE data points that passed all quality flags ranged from 43 to 1324 days . However, calculated flux isotope ratios that did not pass quality flags might not necessarily be low quality. For example, atmospheric conditions could be such that a two-member mixing model does not work. In this case, the raw data would be of high quality, but the Miller-Tans mixing model method would fail. Future work could investigate other methods besides the Miller-Tans method as to increase the number of data points that pass imposed quality flags.The site averaged (±standard deviation) all time isotope flux values ranged from −252.4 (72.8) ‰ to −51.9 (8.9) for δNEE Fig. . The sit22 at 27 globally distributed sites from the International Atomic Energy (IAEA) Global Network of Isotopes in Precipitation (GNIP) database26. Downscaling biweekly observations to daily estimates cannot produce the true value (as these are unknown) but only statistically representative realizations. We quantified the uncertainty associated with the average of the daily precipitation estimates by calculating the standard deviation of each precipitation event across an ensemble of 100 precipitation time series at each site . Depending on the application of the daily isotope ratios, generating an ensemble of daily time series may be advantageous to capture impacts of the standard error as a function of time or space. To generate ensemble sets, the provided python script, which generates a single random realization of the daily precipitation water isotope data product, can be run numerous times.The implemented statistical downscaling method was previously validatedite Fig. . The aveet al.13 (with a bias of 0.11‰ and precision of ~0.4‰) and here similar procedures were used for the atmospheric water vapor ratios. Within the context of a simplified mixing model source estimation approach, uncertainty in the estimation of the isotope composition of surface-atmosphere fluxes is associated with accuracy of isotope measurements themselves and the range of atmospheric carbon dioxide and water vapor concentrations observed during the averaging interval32. The average of the daily standard error in the mixing model regression slope is shown in Fig. FET and Fig. FNEE. These standard errors represent the uncertainty associated with estimation of the isotope ratio of the FET and FNEE within the mixing model framework. The mean of the standard error of the Miller-Tans model slope across all sites ranged from 1.3 to 3.5 (+/− 0.6) for δ2H, from 0.2 to 0.5 (+/− 0.1) for δ18O, and from 0.2 to 0.7 (+/− 0.1) for δ13C.The generation of calibrated isotope ratios associated with the atmospheric carbon dioxide flux were validated by Fiorella FP are contained in four CSV files containing time series of δ2H (“daily_p_d2H_mean.csv”), δ2H (“daily_p_d2H_std.csv”), δ18O (“daily_p_d18O_mean.csv”), and δ18O (daily_p_d18O_std.csv) ratios corresponding to observed precipitation derived from the 100 ensemble members. A corresponding metadata file (“daily_p_metadata.csv”) describing the quality of each data point is provided as well are the 100 timeseries. The script “Estimate_daily_p_flux_iso.py” will implement the complete statistical downscaling method22 with 100 ensemble runs and save daily water isotope time series (mean and standard deviation) which correspond to observed daily precipitation amounts at each NEON site to comma-separated values (CSV) files. The user can change which site data are analyzed and how many ensembles are generated. The user can change the date range they wish to look at. Additionally, the user can change the “precip_filter” variable to change the magnitude of the precipitation events which will be filtered out of the downscaling method and consequently update the output time series and metadata files.Water isotope ratios of FET and carbon isotope ratios of the FNEE are contained in three CSV files per time-window , where “xxx” is the time-window . Each are associated with a corresponding error and metadata file  describing the error estimate of the flux and the quality of each data point. NEON isotope atmospheric isotope ratios were calibrated using the NEONiso package and the FET and FNEE flux isotope composition estimation procedure for δ13C, δ2H, and δ18O were done by leveraging the R scripts “mixing_model_d13C.R”, “mixing_model_d2H.R”, and “mixing_model_d18O.R”, respectively. The script “et_nee_flux.py” takes the output from the mixing-model scripts, reshapes, and assigns quality flags to the data. For the quality flags of FET and FNEE fluxes, the user can adjust the minimum number of the data points used in the Miller-Tans mixing model, adjust the threshold value of the R2 regression, and implement other filters besides the IQR filtering.Water isotope ratios of the