Rainfall received by river basins is partitioned into blue water (surface and groundwater) and green water (soil water used by plants), largely determining how much water is available for terrestrial productivity, aquatic ecosystems, and human water use1,2. Environmental properties, such as aridity, are known to influence blue-green partitioning through evapotranspiration losses to the atmosphere; yet the imprint of climate on the blue–green water cycling within local catchments across river networks remains unknown. Here, we combine stable‑water‑isotope observations with a mass‑balance model for all 20,852 sub-catchments of the Mississippi River Basin and show that climate coupling—defined as the degree to which blue–green water cycling tracks aridity—decreases predictably with river‑network position. Headwaters partitioning fluxes and are more tightly coupled to climate, whereas downstream catchments converge toward similar partitioning states and systematically decouple from aridity. Because the Mississippi River Basin captures ~80% of Earth’s major climate–landform combinations outside tropical and polar extremes, this network‑position scaling provides a transferable constraint on where climate change and water use are most likely to perturb terrestrial water cycling across river basin.

The past four decades have witnessed a strengthening of the winter anticyclonic circulation over the Barents-Kara Sea (BKS), a change that has contributed substantially to amplified local warming and sea ice loss, as well as to Eurasian cooling. However, the cause of this trend in the BKS atmospheric circulation remains unknown. Here we show that anthropogenic greenhouse gases are the primary driver of the strengthening of the BKS anticyclonic circulation, with anthropogenic aerosols playing a secondary role, both together accounting for about 86% of the observed circulation trend. Both forcings induce an amplified BKS low-tropospheric warming through coupling with strong sea ice loss. This amplified warming raises geopotential height aloft through thermal expansion, causing an anomalous anticyclonic anomaly, which in turn enhances warming and sea ice loss, forming a positive feedback loop. Our work provides a theoretical framework for understanding Arctic atmospheric circulation responses to anthropogenic warming and may have implications for climate and environment in the Arctic and beyond.

Paleo-CO2 reconstructions are integral to understanding the evolution of Earth system processes and their interactions given that atmospheric CO2 concentrations are intrinsically linked to planetary function. Furthermore, past periods of major climate change provide unique insights into the response of land-atmosphere-ocean interactions to warming-induced climate change, particularly for times of pCO2 comparable to those projected for our future. How well the past can inform the future, however, depends on how well paleo-CO2 estimates areconstrained. CO2 estimates exist for much of the past half-billion years (the Phanerozoic), but proxies differ in their assumptions and degree of understanding, and there is substantial uncertainty and inconsistency in existing paleo-CO2 estimates. Here, we introduce a community initiative, CO2PIP, focused on advancing the science of paleo-CO2 reconstruction through critically evaluating and modernizing existing records and building a statistically robust multi-proxy atmospheric CO2 record for the Phanerozoic. CO2PIP builds on the previous work of the Cenozoic CO2 Proxy Integration Project (CenCO2PIP) Consortium (Hönisch et al., 2023) and takes a multi-step approach to building the next generation Phanerozoic CO2 record. We are building a standardized paleo-CO2 proxy data repository that includes all metadata and updated chronology and meets FAIR (findable, accessible, interoperable, reusable) data standards. Existing terrestrial-based CO2 estimates are being modernized through additional analyses and measurements, and a set of forward proxy system models are being developed to provide a quantified representation of proxy sensitivities to environmental and ecophysiological conditions and processes that govern the CO2 signals. Ultimately, statistical inversion analysis of the simulated and modernized proxy datasets will be used to produce quantitative, data-driven CO2 reconstructions for individual records and to generate a robust, quantitative reconstruction of atmospheric CO2 concentrations through the Phanerozoic. Digital infrastructure for presenting and archiving the CO2 compilation and project outputs (https://paleo-co2.org/) ensures full accessibility to the scientific community and the public.Hönisch, B. Royer, D., Breecker, D. O., et al., 2023, Towards a Cenozoic history of atmospheric CO2. Science, v. 382 (6675), DOI: 10.1126/science.adi5177).

Quantifying relative contributions of plant transpiration ( T ) and soil evaporation to evapotranspiration ( ET ) is crucial to better understand how vegetation influences and controls ET , the largest efflux of the terrestrial water balance. Here, we derive estimates of transpiration fraction ( T/ET ) using consistent isotope‐based ET partitioning methods for 13 sites spanning five ecosystem types of the continental US, capturing 56 snapshots of T/ET during the growing season. We found transpiration dominated the ET flux across all sites with a mean T/ET of 0.81 ± 0.08 (±standard error). Sites and dates with higher vegetation indices exhibited higher T/ET and transpiration rates, with the latter increasing 0.30 mm/day per unit Leaf Area Index and 2.9 mm/day per unit Normalized Difference Vegetation Index. Counter to expectations, antecedent precipitation had no effect on T/ET . Despite the breadth of ecosystems and conditions represented, evaporation exceeded transpiration only once, suggesting that evaporation rarely dominates ET in the growing season.

Carbon and oxygen isotope ratios of human tooth enamel carbonate are frequently used for reconstruction of past diet, movement, and environmental conditions. Despite a long legacy of research, samples are prepared and analyzed using a remarkably broad range of protocols, and this methodological heterogeneity raises questions about the comparability of isotopic data across studies. We report a systematic comparison of isotope delta (delta) values for 10 "modern" faunal teeth (obtained from field recoveries) measured in two different laboratories. Our tests included comparisons of enamel powder subsamples that were chemically pretreated using commonly adopted protocols and subsamples that received no pretreatment. We also evaluated delta values generated with and without (1) standardizing the reaction temperature used for sample acidification and (2) baking the samples and vials to remove moisture before analysis. The results showed that delta values from the two laboratories were systematically different when samples were chemically pretreated, but that differences were smaller or negligible for untreated samples. Standardization of acid reaction temperature and baking also improved comparability. We suggest that the widely adopted practice of chemical pretreatment of enamel samples is largely unnecessary and may compromise the accuracy of stable isotope analyses. Technology Readiness Level: TRL 4.

Species distributions depend on fine-scale ecological processes and population growth trajectories and are influenced by climate and weather changes. However, the characterization of inter-population dynamics underlying the geographic distributions of migratory organisms remains challenging. We adopted a stable isotope approach to investigate the dynamic population geography of a terrestrial migratory bird across multiple generations. We found that the age-specific geographic source of Mountain Plovers sampled during winter shifted over four years across a latitudinal gradient. Moreover, our results show that differential effects of climate on the probability of occurrence at the wintering ground could be a driver of population turnover in a migratory species adapted to extreme environmental stochasticity (i.e., drought occurrence). We propose a framework for the identification of spatial and temporal climate and weather components and respective effects on population composition and recruitment into migratory wintering populations. Our approach is useful to reveal population compositional shifts through hydrogen stable isotope analysis while accounting for cumulative drought effects.

Mass spectrometry and laser spectroscopy have been widely employed for precise water vapor isotope measurements. Nevertheless, these techniques are limited by logistical challenges in fieldwork, consequently constraining the temporal and spatial resolution of measurements. Specifically, water vapor isotope measurements are primarily limited to near-surface levels, while measurements in the free troposphere are notably scarce. Portable sampling devices, such as air bags and glass bottles, have therefore become necessary alternatives for collecting, storing, and transporting gaseous samples in diverse environments prior to analysis with less portable instruments. In drone-based high-altitude vapor sampling, air bags are preferred for their lighter weight and greater flexibility compared to glass bottles. Nevertheless, they present specific challenges, such as potential sample contamination and isotopic fractionation during storage, primarily due to the inherent permeability of air bags. Here, we developed a theoretical model for water vapor diffusion through the sampling bag surface, with parameters calibrated through laboratory experiments. This model enables the reconstruction of the initial isotopic composition of sampled vapor based on measurements obtained within the bag and from the surrounding environment. This diffusion model underwent rigorous validation through experiments conducted under varying humidity and isotopic composition differences between the inside and outside of the air bag, confirming its reliability. We applied this correction method to air samples collected at various pressures up to the upper troposphere using an air bag-mounted drone that we developed, thereby estimating the initial isotopic composition and uncertainty based on our observations. Our correction method enhances the reliability and applicability of water vapor isotope observations conducted using drones equipped with air bags, and provides a detailed assessment of all potential sources of error and quantifies the uncertainty range of the observations. This approach leverages the strengths of drone-based air bag sampling while mitigating its limitations, thus facilitating the convenient collection of isotopic data throughout the troposphere.

Billions of people rely upon groundwater for drinking water and agriculture, yet predicting how climate change may affect aquifer storage remains challenging. To gain insight beyond the short historical record, we reconstruct changes in groundwater levels in western North America during the last glacial termination (LGT, ~20 to 11 thousand years ago) using noble gas isotopes. Our reconstructions indicate remarkable stability of water table depth in a Pacific Northwest aquifer throughout the LGT despite increasing precipitation, closely matching independent Earth system model (ESM) simulations. In the American Southwest, ESM simulations and noble gas isotopes both suggest a pronounced LGT decline in water table depth in in response to decreasing precipitation, indicating distinct regional groundwater responses to climate. Despite the hydrologic simplicity of ESMs, their agreement with proxy reconstructions of past water table depth suggests that these models hold value in understanding groundwater dynamics and projecting large-scale aquifer responses to climate forcing.

Life-history diversity has been shown to contribute to the resilience of species but can be challenging to quantify, particularly where intra-population genetic structure is lacking. Such is the case for salmon within many fisheries of the North American Pacific Northwest, where the resolution of genetic markers is variable and limited. For Sockeye salmon (Oncorhynchus nerka) within the U.S.-Canada transboundary Taku Watershed, for example, single-nucleotide polymorphisms have successfully distinguished populations associated with specific inland lakes but allocates many individuals to an undifferentiated “River Type” stock. The extent and dynamics of geographic structure within this stock, and thus its potential contribution to the fishery’s resilience, remain unresolved. In such cases, intrinsic non-genetic markers that record key aspects of life history, such as the isotope ratios of body tissues, can provide valuable information on population structure and diversity. We combined a recently published stream network model for strontium stable isotopes (87Sr/86Sr) with otolith (ear stone) microchemistry data to infer the geographic natal origins of 45 adult fish captured during the 2019 run. Our analysis was implemented in a Bayesian framework and leveraged radio tag data as a source of prior information. We distinguish 4 previously undifferentiated sub-populations within the River Type stock, characterized by groups of fish with distinct natal 87Sr/86Sr values and, by inference, natal habitat locations. Although data from additional years will be needed to assess the persistence of these patterns, the result implies potential for previously unrecognized geographic structure within the River Type stock as a contributor to resilience within the population. The lack of genetic differentiation among the subpopulations may suggest that plasticity of habitat use is prevalent and contributes to adaptation. Alternatively, individuals may exhibit strong site fidelity, but differentiation of these sub-populations may be relatively recent or obscured by gene flow. Distinction between these hypotheses should be resolvable by applying the Sr-isotope method to fish recovered across multiple years.

Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of incrementally grown tissues have been used to study movement and migration of animals. Despite advances in characterizing ⁸⁷Sr/⁸⁶Sr turnover [1], the 2-D geometry of turnover in the tooth enamel is still poorly understood. The relocation of a zoo elephant (Loxodonta africana) named Misha provided an exceptional case study for understanding this pattern [1]. We documented the ⁸⁷Sr/⁸⁶Sr turnover in Misha’s molar using high-resolution in situ measurements with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).We prepared a longitudinally-cut thick section from Misha’s molar plate for LA-ICP-MS analysis. Within the tooth enamel, we measured 10 LA-ICP-MS transects parallel to the enamel dentine junction (EDJ), to map the 2-D pattern of ⁸⁷Sr/⁸⁶Sr turnover. Within the dentine, we measured a transect adjacent to the EDJ to document the unattenuated ⁸⁷Sr/⁸⁶Sr turnover sequence. We also analyzed conventionally drilled enamel samples from the same molar plate using the solution method for ⁸⁷Sr/⁸⁶Sr to document any turnover signal attenuation.Molar dentine data are consistent with the published Sr turnover pattern in Misha’s tusk dentine. The inner half of the molar enamel preserves the turnover features in high fidelity, with a 2-D turnover geometry closely following that of enamel apposition. By contrast, the middle to outer surface of the enamel shows progressively more elevated ⁸⁷Sr/⁸⁶Sr values than those of the dentine. Data from drilled enamel samples show an attenuated turnover pattern due to averaging during drilling, as well as more elevated ⁸⁷Sr/⁸⁶Sr. We attribute these elevated Sr ratios to post-relocation Sr overprinting primarily on the outer enamel surface during enamel maturation.Our results suggest that in situ LA-ICP-MS analysis of the inner half of enamel best recovers the time scale and magnitude of the ⁸⁷Sr/⁸⁶Sr turnover in an elephant molar. By contrast, the attenuated and overprinted turnover sequence from conventionally drilled enamel samples may lead to biased interpretations of the timing and geospatial scale of the animal’s movement history. To properly interpret conventionally drilled enamel sequences, future work would benefit from a modeling framework that can account for attenuation, overprint, and turnover of Sr, to quantitatively reconstruct movement or life history of extant and extinct animals. References:[1] Yang, D., Bowen, G. J., Uno, K. T., Podkovyroff, K., Carpenter, N. A., Fernandez, D. P., & Cerling, T. E. (2023). BITS: A Bayesian Isotope Turnover and Sampling model for strontium isotopes in proboscideans and its potential utility in movement ecology. Methods in Ecology and Evolution, 14, 2800–2813. https://doi.org/10.1111/2041-210X.14218

RATIONALE:The isotopic composition of foods is useful for verifying origin and provides baseline information for interpreting isotopic data from human tissues in dietary and forensic research. Despite their widespread consumption, baby foods in the United States remain isotopically understudied. This study presents and analyzes an exploratory δ2H, δ18O, δ13C, and δ15N dataset for pureed baby foods spanning different food types, labeling designations, brands, and geographic purchase locations. METHODS:Pureed baby food was collected from across the United States, focusing on three widely consumed food types (banana, carrot, and sweet potato, n = 117) across several commercial brands. Cryogenically extracted water and residual dry solids were analyzed to determine the δ2H and δ18O values of water and δ13C and δ15N values of bulk solids. RESULTS:We found significant isotopic differences between food types, with bananas having the highest δ2H, δ18O, and δ13C values and carrots the lowest. These offsets persisted across different brands. No significant differences were observed across purchase locations. δ15N values were significantly higher for bananas and carrots labeled "organic," and d-excess and δ18O values in banana and sweet potatoes were significantly higher for foods containing added water. CONCLUSIONS:Our dataset documents isotopic patterns in pureed baby foods aligning with expectations related to differences in growing environment, farming practices (e.g., organic), and processing (e.g., added water). This supports the potential utility of stable isotope data for studying and authenticating baby food production and distribution. Although limited in sample size, the absence of geographic isotopic variation, which is consistent with geographic homogenization of the modern "supermarket diet," suggests isotopic inputs from food to bodies of infants and children consuming these prepared foods depend more on consumer choices than on location. Thus, early childhood isotope data may be more informative for reconstructing diet than residence.

Global climate change is projected to lead to an expansion of wildfire-prone regions coupled with increase in fire frequency and intensity. In the Western US the characteristic fire season has changed considerably in recent years. Notably, there has been an observed doubling of autumn fire activity, an increase in the occurrence and duration of extreme fire weather conditions, and an intensification of wildfire severity. This trend is expected to persist and intensify with increased warming. For California (CA), climate models predict minimal changes to mean annual precipitation, but a substantial increase in climate volatility on the decadal and sub-centennial scale. This volatility manifests as persistent droughts interrupted by pluvial episodes, creating what is referred to as a 'whiplash climate.' However, predicting how the wildfire regime in CA will evolve with increased climate volatility remains a challenging task.StalFire is multi-lab collaborative framework (consortium) established to facilitate research focused on utilizing paleoclimate data archived in stalagmites. The primary goal is to provide new insights into paleo-wildfires, with a specific emphasis on assessing the paleoclimate-wildfire relationship in CA across multiple timescales and under different climate states. Drawing upon over a decade of monitoring and stalagmite record development in CA caves exposed to significant fires, this consortium project is guided four research objectives: (1) Monitoring tracer evolution across karst systems to improve conventional proxies and expand the understanding and application of promising new qualitative hydroclimate proxies and fire tracers. (2) Developing aquantified representation — a forward proxy system model — of the conditions and processes that govern the different proxy signals of past hydroclimate and fire behavior in stalagmites. (3) Expanding and improving existing CA multi-proxy records for stalagmites representing transects of the north-south climate dipole extending from coastal to Sierra Nevada regions. 4) Conducting site-specific to regional proxy-model comparisons and providing quantitative reconstructions of hydroclimate and fire activity in CA over the past 70,000 years. We provide a summary of initial results of the StalFire Consortium that focus on developing a 'surface-to-stalagmite' understanding of hydroclimate and fire tracers through monitoring coupled with proxy development (d44Ca, fluid inclusion water isotopes, dual clumped isotope, and plant/microbial biomarkers, biomass burning-derived organic molecules like anhydrosugars and polycyclic aromatic hydrocarbons). Finally, we aim for this presentation to encourage discussion among researchers, labs, and work groups involved in paleo-fire coupled with hydroclimate research, from method development to paleorecord analysis.

The intensification of the hydrological cycle, driven by global warming is expected to amplify extreme weather events and associated erosion. These hydrological shifts are likely to disrupt soil stability, accelerate organic carbon mineralization, and alter terrestrial ecosystems, all of which have potential implications for carbon cycle dynamics. Such terrestrial carbon cycle feedback mechanisms remain poorly constrained. While relatively slow compared to present-day carbon cycle change, the millennial scale onset of the Paleocene-Eocene Thermal Maximum (PETM, ~56 million years ago) was similarly associated with a massive input of 13C-depleted carbon into the ocean-atmosphere system that is recorded by a negative carbon isotope excursion (CIE) in sedimentary components. The PETM marks a global temperature increase of ~5 °C and is characterized by significant associated hydrological disturbances, erosion, and vegetation changes; the precise timing, impact and spatial scale of these processes are still being debated. Here, we show vegetation shifts at the Norwegian Margin during the CIE onset interval at centennial scale resolution. Furthermore, we show terrestrial disturbances during the CIE onset are synchronous along several continental margins globally based on organic microfossil assemblages and reworked soils (clay minerals and organic matter). These observations signal changes in terrestrial biomass, intensified oxidation of soils and weathering of fossil organic carbon, potentially acting as a positive carbon cycle feedback mechanism. Carbon cycle model simulations indicate that these shifts in terrestrial carbon storage and fluxes may have appreciably contributed to the CIE and climate change, highlighting the importance of constraining the response of terrestrial biosphere feedback mechanisms to changing weather and climate.

The Paleocene Epoch represents a transitional Earth system state featuring climatic relaxation between the extreme warmth of the Late Cretaceous and Early Eocene. Carbon isotope and sedimentological data have been invoked as evidence for elevated organic carbon burial and CO2 drawdown throughout the early Paleocene, constituting a potential driver of and/or feedback on climate change. Despite this, quantitative proxy reconstructions of Paleocene atmospheric CO2 concentrations have remained sparse, limiting our ability to test hypotheses for the role of carbon cycle change in Paleocene Earth system change. Here we produce quantitative CO2 reconstructions spanning the Paleocene by combining data from marine (foraminiferal calcite) and terrestrial (pedogenic carbonate) proxy systems. We integrate data from these proxy systems, together with complementary paleo-environmental proxy data, using newly developed proxy system models implemented within the Bayesian Joint Proxy Inversion (JPI) framework. Although each individual proxy system is under-constrained, the combination of information from distinct systems and constraints provided by ancillary data produces a coherent, well-resolved paleo-CO2 reconstruction. The record suggests strong coupling between carbon cycle processes, atmospheric CO2 levels, and climate throughout the Paleocene. Integration of a simple carbon cycle model driven by changing sedimentary organic carbon burial within the JPI analysis provides additional constraints on the CO2 reconstruction and demonstrates that this process is generally consistent with the available proxy evidence.

Strontium isotope ratios (87Sr/86Sr) in dental tissues are widely used to study animal and human migration. However, questions remain regarding how different biological processes and sampling methods influence measured 87Sr/86Sr and subsequent interpretations. We present a unique experiment with the known relocation history of a zoo elephant to evaluate the influence of biological turnover, tissue type, and sampling methods. We collected 87Sr/86Sr data from the elephant's tusk and molar enamel using in-situ laser ablation microsampling, conventional drilling, and micromilling techniques. Our data comparisons show that the innermost enamel best records the primary 87Sr/86Sr turnover history, while enamel maturation affects outer enamel 87Sr/86Sr. Conventional drilling and micromilling show attenuated 87Sr/86Sr records due to enamel maturation and sample averaging. To effectively account for 87Sr/86Sr signal attenuation due to turnover, we demonstrated a microsampling-modeling workflow using our tusk and enamel data. This recommended workflow can facilitate geospatially explicit interpretations of seasonal migration.

The use of isotopic analysis for human mobility, geolocation, and forensic identification has become common over the past two decades, yet its effectiveness depends on the availability of well-documented reference data. Many reference data exist in the literature, but the suitability of these data for forensic applications has not been critically assessed. Here, we compile oxygen and strontium isotope data for North American human tissues (hair, nails, bone, and tooth enamel). We review the geographic distribution of these data, evaluate their relationship with the predicted geographic variation, and assess potential sources of bias that may limit the comparability of different datasets in the compilation. A substantial number of data are available for some of these substrates and isotope systems, but in most cases, their geographic distribution is patchy with many areas under sampled. Except for hair Sr isotopes, a robust relationship occurs between human tissue values and modeled local environmental values, suggesting theoretically expected relationships between human tissues and local sources of O and Sr are expressed. These relationships are noisy; we identify several methodological differences that produce systematic biases across the compiled data. Based on these findings, we suggest interpreting forensic isotope data using currently published reference data is problematic. We propose the adoption of consistent metadata reporting and standardized laboratory protocols to enhance the utility of data gathered in future research as these practices could lead to measurable improvements in the strength of forensic interpretations derived from human tissue isotope data.

RationaleMany insect species undertake multigenerational migrations in the Afro‐tropical and Palearctic ranges, and understanding their migratory connectivity remains challenging due to their small size, short life span and large population sizes. Hydrogen isotopes (δ2H) can be used to reconstruct the movement of dispersing or migrating insects, but applying δ2H for provenance requires a robust isotope baseline map (i.e. isoscape) for the Afro‐Palearctic.MethodsWe analyzed the δ2H in the wings (δ2Hwing) of 142 resident butterflies from 56 sites across the Afro‐Palearctic. The δ2Hwing values were compared to the predicted local growing‐season precipitation δ2H values (δ2HGSP) using a linear regression model to develop an insect wing δ2H isoscape. We used multivariate linear mixed models and high‐resolution and time‐specific remote sensing climate and environmental data to explore the controls of the residual δ2Hwing variability.ResultsA strong linear relationship was found between δ2Hwing and δ2HGSP values (r2 = 0.53). The resulting isoscape showed strong patterns across the Palearctic but limited variation and high uncertainty for the Afro‐tropics. Positive residuals of this relationship were correlated with dry conditions for the month preceding sampling whereas negative residuals were correlated with more wet days for the month preceding sampling. High intra‐site δ2Hwing variance was associated with lower relative humidity for the month preceding sampling and higher elevation.ConclusionThe δ2Hwing isoscape is applicable for tracing herbivorous lepidopteran insects that migrate across the Afro‐Palearctic range but has limited geolocation potential in the Afro‐tropics. The spatial analysis of uncertainty using high‐resolution climatic data demonstrated that many African regions with highly variable evaporation rates and relative humidity have δ2Hwing values that are less related to δ2HGSP values. Increasing geolocation precision will require new modeling approaches using more time‐specific environmental data and/or independent geolocation tools.

The forensic application of stable oxygen isotope data from human tissues depends on naturally occurring isotopic variation in drinking water across geographic areas. One factor which complicates interpretation of forensic data is local variability: if a wide range of drinking water values is in a small geographic region it may be difficult to identify or rule out that region as a location of origin. We examine data from community collection programs documenting tap water isotope variation within 30 cities\developed areas throughout the United States. Isotopic variation within individual developed areas ranged widely, from essentially nil to greater than 9‰ (δ18O interdecile range). Many (14/30) of the study areas exhibited multi-modal isotope distributions, even in cases where the isotopic range was very small (e.g., <2.0‰), suggesting that the use of multiple, isotopically different sources was common. Most variation was attributed to differences in the source-region altitude or degree of evaporation for different water sources, and we observe limited variation in areas where contrasts in these variables are small. Variation tended to be largest in cities across the western USA. We explored correlations between the magnitude of local isotopic variation and climatic, environmental, and socioeconomic variables. We found that higher levels of variation occurred at sites where local water resources were likely to be more isotopically heterogeneous and in lower supply, consistent with the development of supply systems in these areas that access diverse and isotopically heterogeneous water resources. We also found that variation was positively correlated with larger developed areas, suggesting that pressures related to city growth may affect the degree to which infrastructure tapping diverse water resources is developed. Collectively, our results suggest that the influence of local tap water isotope heterogeneity on the precision of forensic geolocation is systematic and somewhat predictable, information to be factored into future applications.

The late Paleocene and early Eocene (LPEE) are characterized by long-term (million years, Myr) global warming and by transient, abrupt (kiloyears, kyr) warming events, termed hyperthermals. Although both have been attributed to greenhouse (CO 2 ) forcing, the longer-term trend in climate was likely influenced by additional forcing factors (i.e., tectonics) and the extent to which warming was driven by atmospheric CO 2 remains unclear. Here, we use a suite of new and existing observations from planktic foraminifera collected at Pacific Ocean Drilling Program Sites 1209 and 1210 and inversion of a multiproxy Bayesian hierarchical model to quantify sea surface temperature (SST) and atmospheric CO 2 over a 6-Myr interval. Our reconstructions span the initiation of long-term LPEE warming (~58 Ma), and the two largest Paleogene hyperthermals, the Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma) and Eocene Thermal Maximum 2 (ETM-2, ~54 Ma). Our results show strong coupling between CO 2 and temperature over the long- (LPEE) and short-term (PETM and ETM-2) but differing Pacific climate sensitivities over the two timescales. Combined CO 2 and carbon isotope trends imply the carbon source driving CO 2 increase was likely methanogenic, organic, or mixed for the PETM and organic for ETM-2, whereas a source with higher δ 13 C values (e.g., volcanic degassing) is associated with the long-term LPEE. Reconstructed emissions for the PETM (5,800 Gt C) and ETM-2 (3,800 Gt C) are comparable in mass to future emission scenarios, reinforcing the value of these events as analogs of anthropogenic change.

Within the front end of the nuclear fuel cycle, many processes impart forensic signatures. Oxygen-stable isotopes (delta O-18 values) of uranium-bearing materials have been theorized to provide the processing and geolocational signatures of interdicted materials. However, this signature has been minimally utilized due to a limited understanding of how oxygen isotopes are influenced during uranium processing. This study explores oxygen isotope exchange and fractionation between magnesium diuranate (MDU), ammonium diuranate (ADU), and uranyl fluoride (UO2F2) with steam (water vapor) during their reduction to UOx. The MDU was precipitated from two water sources, one enriched and one depleted in O-18. The UO2F2 was precipitated from a single water source and either directly reduced or converted to ADU prior to reduction. All MDU, ADU, and UO2F2 were reduced to UOx in a 10% hydrogen/90% nitrogen atmosphere that was dry or included steam. Powder X-ray diffraction (p-XRD) was used to verify the composition of materials after reduction as mixtures of primarily U3O8, U4O9, and UO2 with trace magnesium and fluorine phases in UOx from MDU and UO2F2, respectively. The bulk oxygen isotope composition of UOx from MDU was analyzed using fluorination to remove the lattice-bound oxygen, and then O-2 was subsequently analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope compositions of the ADU, UO2F2, and the resulting UOx were analyzed by large geometry secondary ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU, ADU, and UO2F2 experienced significant oxygen isotope exchange, and the resulting delta O-18 values of UOx approached the values of the steam. When reduced without steam, the delta O-18 values of converted ADU, U3O8, and UOx products remained similar to those of the UO2F2 starting material. LG-SIMS isotope mapping of F impurity abundances and distributions showed that direct steam-assisted reduction from UO2F2 significantly removed F impurities while dry reduction from UO2F2 led to the formation of UOx that was enhanced in F impurities. In addition, when UO2F2 was processed via precipitation to ADU and calcination to U3O8, F impurities were largely removed, and reductions to UOx with and without steam each had low F impurities. Overall, these findings show promise for combining multiple signatures to predict the process history during the conversion of uranium ore concentrates to nuclear fuel.