The end-Triassic extinction is characterized by major losses in both terrestrial and marine diversity, setting the stage for dinosaurs to dominate Earth for the next 136 million years. Despite the approximate coincidence between this extinction and flood basalt volcanism, existing geochronologic dates have insufficient resolution to confirm eruptive rates required to induce major climate perturbations. Here, we present new zircon uranium-lead (U-Pb) geochronologic constraints on the age and duration of flood basalt volcanism within the Central Atlantic Magmatic Province. This chronology demonstrates synchroneity between the earliest volcanism and extinction, tests and corroborates the existing astrochronologic time scale, and shows that the release of magma and associated atmospheric flux occurred in four pulses over about 600,000 years, indicating expansive volcanism even as the biologic recovery was under way.T he approximate temporal coincidence between the five major extinction events over the past 542 million years and the eruption of large igneous provinces (LIPs) has led to speculation that environmental perturbations generated by the emplacement of large volumes of magma and associated outgassing over short periods of time triggered each global biologic crisis (1). Establishing an exact link between extinctions and LIP eruptions has proved difficult because of the geographic separation between LIP volcanic deposits and stratigraphic sequences preserving evidence of the extinction. In most cases, uncertainties on radioisotopic dates used to correlate between geographically separated study areas exceed the duration of both the extinction interval and LIP volcanism by an order of magnitude. This hinders evaluation of any relationship between magmatism and extinction and precludes accurate estimates of volcanic effusion rates, associated volatile release, and extinction mechanisms.The end-Triassic extinction (ETE)-marked within early Mesozoic basins of eastern North America by a dramatic turnover in fossil pollen, spores (sporomorphs), and vertebrates (2)-is one of the largest Phanerozoic mass extinctions, occurring just before the Triassic-Jurassic boundary (3, 4), and has long been thought to be associated with the eruptions of the Central Atlantic Magmatic Province (CAMP) (5, 6). CAMP is the most aerially extensive LIP on Earth, and with volume estimates between 2-3 × 10 6 km 3 , it ranks as one of the most voluminous (7) (Fig. 1). Remnants of CAMP are found on four continents and consist primarily of continental thoeliitic basalts emplaced as subaerial flows and intrusive bodies during rifting of the Pangean supercontinent and incipient formation of the Atlantic Ocean basin (Fig. 1) fig. S1) prevents estimation of the volume of magma erupted over unit time, a critical factor for evaluating extinction mechanisms such as CO 2 -induced global warming (12, 13) ocean acidification (14, 15), or sulfur aerosol-induced "volcanic winters" (16). U-Pb Geochronology of CAMP Flows and IntrusivesHere, we present zirco...

The LA‐ICP‐MS U‐(Th‐)Pb geochronology international community has defined new standards for the determination of U‐(Th‐)Pb ages. A new workflow defines the appropriate propagation of uncertainties for these data, identifying random and systematic components. Only data with uncertainties relating to random error should be used in weighted mean calculations of population ages; uncertainty components for systematic errors are propagated after this stage, preventing their erroneous reduction. Following this improved uncertainty propagation protocol, data can be compared at different uncertainty levels to better resolve age differences. New reference values for commonly used zircon, monazite and titanite reference materials are defined (based on ID‐TIMS) after removing corrections for common lead and the effects of excess 230Th. These values more accurately reflect the material sampled during the determination of calibration factors by LA‐ICP‐MS analysis. Recommendations are made to graphically represent data only with uncertainty ellipses at 2s and to submit or cite validation data with sample data when submitting data for publication. New data‐reporting standards are defined to help improve the peer‐review process. With these improvements, LA‐ICP‐MS U‐(Th‐)Pb data can be considered more robust, accurate, better documented and quantified, directly contributing to their improved scientific interpretation.

One sentence summary:238 U/ 235 U ratios for U-bearing accessory minerals from a diverse suite of terrestrial rocks indicate a >5‰ range with an average zircon value of 238 U/ 235 U = 137.818 ± 0.045 (2σ) which is consistent with the composition of other terrestrial and meteoritic reservoirs. 2, 3). However, a robust λ 235 U can only be determined with U-Pb analyses using a tracer calibration that is traceable to SI units and free of other potential sources of bias, so we refrain from suggesting this value be adopted at present and urge caution in abandoning the Jaffey et al (1) λ 235 U determination until such a dataset has been generated and evaluated.An emerging 238 U/ 235 U dataset for a wide range of rocks, minerals, and meteorites is now available (17,18,25,(30)(31)(32)(33) and compiled here (Fig. 3)

[1] High-precision U-Pb geochronology by isotope dilution-thermal ionization mass spectrometry is integral to a variety of Earth science disciplines, but its ultimate resolving power is quantified by the uncertainties of calculated U-Pb dates. As analytical techniques have advanced, formerly small sources of uncertainty are increasingly important, and thus previous simplifications for data reduction and uncertainty propagation are no longer valid. Although notable previous efforts have treated propagation of correlated uncertainties for the U-Pb system, the equations, uncertainties, and correlations have been limited in number and subject to simplification during propagation through intermediary calculations. We derive and present a transparent U-Pb data reduction algorithm that transforms raw isotopic data and measured or assumed laboratory parameters into the isotopic ratios and dates geochronologists interpret without making assumptions about the relative size of sample components. To propagate uncertainties and their correlations, we describe, in detail, a linear algebraic algorithm that incorporates all input uncertainties and correlations without limiting or simplifying covariance terms to propagate them though intermediate calculations. Finally, a weighted mean algorithm is presented that utilizes matrix elements from the uncertainty propagation algorithm to propagate random and systematic uncertainties for data comparison between other U-Pb labs and other geochronometers. The linear uncertainty propagation algorithms are verified with Monte Carlo simulations of several typical analyses. We propose that our algorithms be considered by the community for implementation to improve the collaborative science envisioned by the EARTHTIME initiative.

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[1] In the past decade, major advancements in precision and accuracy of U-Pb geochronology, which stem from improved sample pretreatment and refined measurement techniques, have revealed previously unresolvable discrepancies among analyses from different laboratories. One solution to evaluating and resolving many of these discrepancies is the adoption of a common software platform that standardizes data-processing protocols, enabling robust interlaboratory comparisons. We present the results of a collaboration to develop cyber infrastructure for high-precision U-Pb geochronology based on analyzing accessory minerals by isotope dilution-thermal ionization mass spectrometry. This cyber infrastructure implements an architecture specifying the workflows of data acquisition, statistical filtering, analysis and interpretation, publication, community-based archiving, and the compilation and comparison of data from different laboratories. The backbone of the cyber infrastructure consists of two open-source software programs: Tripoli and U-Pb_Redux.

Mixed 235 U-233 U-205 Pb (-202 Pb) tracers for U-Pb isotope-dilution isotope ratio mass spectrometry have been prepared under the auspices of the EARTHTIME Initiative. The methods and results for the preparation and calibration of the U/Pb ratio and isotopic abundances are given, and the various sources of uncertainty are discussed and quantified. The accuracy of the EARTHTIME U-Pb tracer isotopic composition can be traced back to SI units via a series of assay and isotopic composition reference materials combined with the experiments described herein. The parameters used in calculating U/Pb ratios (and inferentially U-Pb dates) have correlated uncertainties that result in a total uncertainty contribution to 206 Pb/ 238 U dates of ± < 0.03% (95% confidence). For suitable terrestrial materials such as zircon, when other sources of uncertainty have been minimised (e.g., open-system behaviour, 238 U/ 235 U variation, intermediate daughter product disequilibrium, common Pb, etc.) the U-Pb tracer calibration uncertainty is a limiting factor in the accuracy of U-Pb geochronology -but less so than the uncertainty in the 238 U and 235 U decay constants (±0.11 and 0.14% 2r). The calibration approach of the mixed EARTHTIME 235 U-233 U-205 Pb (-202 Pb) tracers, in addition to updated values for reference materials (e.g., mixed gravimetric reference solutions), and parameters (e.g., Pb reference material assay), can be applied to other laboratory-specific U-Pb tracers and will facilitate the generation of accurate and directly inter-comparable U-Pb data.

A statistical approach to evaluating uncertainties in the calibration of multi-element isotopic tracers has been developed and applied to determining the isotopic composition of mixed U-Pb (202 Pb-205 Pb-233 U-235 U) tracers used for accurate isotope dilution U-Pb geochronology. Our experiment, part of the EARTHTIME initiative, directly links the tracer calibration to first-principles measurements of mass and purity that are all traceable to SI units, thereby quantifying the accuracy and precision of U-Pb dates in absolute time. The calibration incorporates new more accurate and precise purity measurements for a number of commonly used Pb and U reference materials, and requires interrelating their isotopic compositions and uncertainties. Similar methods can be used for other isotope systems that utilize multiple isotopic standards for calibration purposes. We also detail the inter-calibration of three publicly available U-Pb gravimetric solutions, which can be used to bring the same first-principles traceability to in-house U-Pb tracers from other laboratories. Accounting for uncertainty correlations in the tracer isotope ratios yields a tracer calibration contribution to the relative uncertainty of a 206 Pb/ 238 U date that is only half of the relative uncertainty in the 235 U/ 205 Pb ratio of the tracer, which was historically used to approximate the tracer related uncertainty contribution to 206 Pb/ 238 U dates. The tracer uncertainty contribution to 206 Pb/ 238 U dates has in this way been reduced to <300 ppm when using the EARTHTIME and similarly calibrated tracers.