1σ Uncertainty Research Articles

Original Research by Young Twinkle Students (ORBYTS): ephemeris refinement of transiting exoplanets

ABSTRACT We report follow-up observations of transiting exoplanets that have either large uncertainties (>10 min) in their transit times or have not been observed for over 3 yr. A fully robotic ground-based telescope network, observations from citizen astronomers, and data from TESS have been used to study eight planets, refining their ephemerides and orbital data. Such follow-up observations are key for ensuring accurate transit times for upcoming ground- and space-based telescopes, which may seek to characterize the atmospheres of these planets. We find deviations from the expected transit time for all planets, with transits occurring outside the 1σ uncertainties for seven planets. Using the newly acquired observations, we subsequently refine their periods and reduce the current predicted ephemeris uncertainties to 0.28–4.01 min. A significant portion of this work has been completed by students at two high schools in London as part of the Original Research By Young Twinkle Students (ORBYTS) programme.

Stability analysis of three exoplanet systems

ABSTRACT The orbital solutions of published multiplanet systems are not necessarily dynamically stable on time-scales comparable to the lifetime of the system as a whole. For this reason, dynamical tests of the architectures of proposed exoplanetary systems are a critical tool to probe the stability and feasibility of the candidate planetary systems, with the potential to point the way towards refined orbital parameters of those planets. Such studies can even help in the identification of additional companions in such systems. Here, we examine the dynamical stability of three planetary systems, orbiting HD 67087, HD 110014, and HD 133131A. We use the published radial velocity measurements of the target stars to determine the best-fitting orbital solutions for these planetary systems using the systemic console. We then employ the N-body integrator mercury to test the stability of a range of orbital solutions lying within 3σ of the nominal best fit for a duration of 100 Myr. From the results of the N-body integrations, we infer the best-fitting orbital parameters using the Bayesian package astroemperor. We find that both HD 110014 and HD 133131A have long-term stable architectures that lie within the 1σ uncertainties of the nominal best fit to their previously determined orbital solutions. However, the HD 67087 system exhibits a strong tendency towards instability on short time-scales. We compare these results to the predictions made from consideration of the angular momentum deficit criterion, and find that its predictions are consistent with our findings.

Utilizing Small Telescopes Operated by Citizen Scientists for Transiting Exoplanet Follow-up

Due to the efforts by numerous ground-based surveys and NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS), there will be hundreds, if not thousands, of transiting exoplanets ideal for atmospheric characterization via spectroscopy with large platforms such as James Webb Space Telescope and ARIEL. However their next predicted mid-transit time could become so increasingly uncertain over time that significant overhead would be required to ensure the detection of the entire transit. As a result, follow-up observations to characterize these exoplanetary atmospheres would require less-efficient use of an observatory’s time—which is an issue for large platforms where minimizing observing overheads is a necessity. Here we demonstrate the power of citizen scientists operating smaller observatories (≤1 m) to keep ephemerides “fresh,” defined here as when the 1σ uncertainty in the mid-transit time is less than half the transit duration. We advocate for the creation of a community-wide effort to perform ephemeris maintenance on transiting exoplanets by citizen scientists. Such observations can be conducted with even a 6 inch telescope, which has the potential to save up to ∼10,000 days for a 1000-planet survey. Based on a preliminary analysis of 14 transits from a single 6 inch MicroObservatory telescope, we empirically estimate the ability of small telescopes to benefit the community. Observations with a small-telescope network operated by citizen scientists are capable of resolving stellar blends to within 5″/pixel, can follow-up long period transits in short-baseline TESS fields, monitor epoch-to-epoch stellar variability at a precision 0.67% ± 0.12% for a 11.3 V-mag star, and search for new planets or constrain the masses of known planets with transit timing variations greater than two minutes.

The 3D non-LTE solar nitrogen abundance from atomic lines

Nitrogen is an important element in various fields of stellar and Galactic astronomy, and the solar nitrogen abundance is crucial as a yardstick for comparing different objects in the cosmos. In order to obtain a precise and accurate value for this abundance, we carried out N I line formation calculations in a 3D radiative-hydrodynamic STAGGER model solar atmosphere in full 3D non-local thermodynamic equilibrium (non-LTE). We used a model atom that includes physically motivated descriptions for the inelastic collisions of N I with free electrons and with neutral hydrogen. We selected five N I lines of high excitation energy to study in detail, based on their strengths and on their being relatively free of blends. We found that these lines are slightly strengthened from non-LTE photon losses and from 3D granulation effects, resulting in negative abundance corrections of around − 0.01 dex and − 0.04 dex, respectively. Our advocated solar nitrogen abundance is log ɛN = 7.77, with the systematic 1σ uncertainty estimated to be 0.05 dex. This result is consistent with earlier studies after correcting for differences in line selections and equivalent widths.

On the XUV Luminosity Evolution of TRAPPIST-1

Abstract We model the long-term X-ray and ultraviolet (XUV) luminosity of TRAPPIST-1 to constrain the evolving high-energy radiation environment experienced by its planetary system. Using a Markov Chain Monte Carlo (MCMC) method, we derive probabilistic constraints for TRAPPIST-1's stellar and XUV evolution that account for observational uncertainties, degeneracies between model parameters, and empirical data of low-mass stars. We constrain TRAPPIST-1's mass to m ⋆ = 0.089 ± 0.001 M ⊙ and find that its early XUV luminosity likely saturated at . From the posterior distribution, we infer that there is a ∼40% chance that TRAPPIST-1 is still in the saturated phase today, suggesting that TRAPPIST-1 has maintained high activity and L XUV/L bol ≈ 10−3 for several gigayears. TRAPPIST-1's planetary system therefore likely experienced a persistent and extreme XUV flux environment, potentially driving significant atmospheric erosion and volatile loss. The inner planets likely received XUV fluxes ∼103–104 times that of the modern Earth during TRAPPIST-1's ∼1 Gyr long pre-main-sequence phase. Deriving these constraints via MCMC is computationally nontrivial, so scaling our methods to constrain the XUV evolution of a larger number of M dwarfs that harbor terrestrial exoplanets would incur significant computational expenses. We demonstrate that approxposterior, an open source Python machine learning package for approximate Bayesian inference using Gaussian processes, accurately and efficiently replicates our analysis using 980 times less computational time and 1330 times fewer simulations than MCMC sampling using emcee. We find that approxposterior derives constraints with mean errors on the best-fit values and 1σ uncertainties of 0.61% and 5.5%, respectively, relative to emcee.

Radiocarbon Dating of Small-sized Foraminifer Samples: Insights into Marine sediment Mixing

ABSTRACTRadiocarbon (14C) can be used to build absolute chronologies and reconstruct ocean ventilation over the last 40 ka. Sample size requirements have restricted14C measurements in marine cores with low foraminifer content, impeding14C-based studies focused on abrupt climate events. Recent developments have demonstrated that small-sized foraminifer samples can now be dated using a gas introduction system at the cost of a small decrease in precision. We explore the potential of gas measurements on benthic and planktonic foraminifers from core SU90-08 (43°03′1″N, 30°02′5″W, 3080 m). Gas measurements are accurate, reproducible within 2σ uncertainty and comparable to graphite measurements. Both techniques yield negative14C benthic-planktonic (B-P) age-offsets after Heinrich event 1. We argue that negative B-P ages result from bioturbation and changes in foraminifer abundances, with the chance of negative B-P especially increased when the14C age gradient between the deep and surface waters is decreased. Small-sized14C measurements seem to capture the variance of the foraminifera age distribution, revealing the active mixing in those archives. Sediment deposition and mixing effects possibly pose a greater obstacle for past14C-based dating and ocean ventilation reconstructions than the measurement precision itself, particularly in relatively low sedimentation rate settings.

Cloud Atlas: High-precision HST/WFC3/IR Time-resolved Observations of Directly Imaged Exoplanet HD 106906b

Abstract HD 106906b is an , ∼15 Myr old directly imaged exoplanet orbiting at an extremely large distance from its host star. The wide separation (7.″11) between HD 106906b and its host star greatly reduces the difficulty in direct-imaging observations, making it one of the most favorable directly imaged exoplanets for detailed characterization. In this paper, we present HST/WFC3/IR time-resolved observations of HD 106906b in the F127M, F139M, and F153M bands. We have achieved ∼1% precision in the lightcurves in all three bands. The F127M lightcurve demonstrates marginally detectable (2.7σ significance) variability with a best-fitting period of 4 hr, while the lightcurves in the other two bands are consistent with flat lines. We construct primary-subtracted deep images and use these images to exclude additional companions to HD 106906 that are more massive than 4 and locate at projected distances of more than ∼500 au. We measure the astrometry of HD 106906b in two HST/WFC3 epochs and achieve precisions better than 2.5 mas. The position angle and separation measurements do not deviate from those in the 2004 HST/ACS/HRC images for more than 1σ uncertainty. We provide the HST/WFC3 astrometric results for 25 background stars that can be used as reference sources in future precision astrometry studies. Our observations also provide the first 1.4 μm water band photometric measurement for HD 106906b. HD 106906b’s spectral energy distribution and the best-fitting BT-Settl model have an inconsistency in the 1.4 μm water absorption band, which highlights the challenges in modeling atmospheres of young planetary-mass objects.

Sensitivity and Uncertainty Analysis of the Maximum Fuel Temperature under Accident Condition of HTR-PM

The maximum fuel temperature under accident condition is the most important parameter of inherently safe characteristics of HTR-PM, and the DLOFC accident may lead to a peak accident fuel temperature. And there are a variety of uncertainty sources in the maximum fuel temperature calculations, and thus the contributions of these uncertainty sources to the final calculated maximum fuel temperature should be quantified to check whether the peak value exceed the technological limit of 1620°C or not. Eight uncertainty input parameters are selected for inclusion in this uncertainty study, and their associated 2 standard deviation uncertainties and probability density functions are specified. Then, the DLOFC thermal analyses and uncertainty analysis are performed with the home-developed ATHENA and CUSA. The numerical results indicate that the pebble-bed effective conductivity and the decay heat contribute the most of the uncertainty in the DLOFC maximum fuel temperature while this peak fuel temperature is most sensitive to the initial reactor power and the decay heat. In short, uncertainties in these selected eight parameters lead to the two standard deviation (2σ) uncertainty of ±77.6°C (or 5.2%) around the mean value of 1493°C for the maximum fuel temperature under DLOFC accident of HTR-PM. At the same time, the LHS-SVDC method of CUSA is recommended to propagate uncertainties in inputs and 100–200 model simulations seem to be sufficient to get an uncertainty prediction with full confidence.

Rate constant of the BrO + BrO reaction over the temperature range 220‐950 K

AbstractThe kinetics of the self‐reaction of BrO radicals has been studied over a wide temperature range (T = 220‐950 K) at 2 Torr total pressure of He using a low‐pressure flow reactor combined with a quadrupole mass spectrometer: BrO + BrO → 2Br + O2 (1a) and BrO + BrO → Br2 + O2 (1b). The overall rate constant for this reaction (defined by d[BrO]/dt = –2k1[BrO]2) was found to be k1 = k1a +k1b = (1.32 ± 0.09) × 10−12 exp(182 ± 22/T) cm3 molecule−1 s−1 at T = 220‐950 K (where the uncertainties represent precision at the 2σ level). The channel‐specific rate constant for the Br2 + O2 forming channel (1b) was determined at T = 220‐450 K: k1b = (2.83 ± 0.35) × 10−13 exp(34 ± 36/T) cm3 molecule−1 s−1 (with 2σ uncertainties). As a result of the comparative analysis of the present data and those from the previous studies, the following Arrhenius expressions are recommended from the present work: k1 = 1.4 × 10−12 exp(200/T) at T = 220‐950 K (independent of pressure at least for T ≥ 250 K) and k1b = 3.2 × 10−13 exp(34/T) cm3 molecule−1 s−1 at T = 220‐450 K and total pressure of 2 Torr, with a conservative estimate of uncertainty of 25%, independent of temperature.

The violent collisional history of aqueously evolved (2) Pallas

Asteroid (2) Pallas is the largest main-belt object not yet visited by a spacecraft, making its surface geology largely unknown and limiting our understanding of its origin and collisional evolution. Previous ground-based observational campaigns returned different estimates of its bulk density that are inconsistent with one another, one measurement1 being compatible within error bars with the icy Ceres (2.16 ± 0.01 g cm−3)2 and the other3 compatible within error bars with the rocky Vesta (3.46 ± 0.03 g cm−3)4. Here we report high-angular-resolution observations of Pallas performed with the extreme adaptive optics-fed SPHERE imager5 on the Very Large Telescope. Pallas records a violent collisional history, with numerous craters larger than 30 km in diameter populating its surface and two large impact basins that could be related to a family-forming impact. Monte Carlo simulations of the collisional evolution of the main belt correlate this cratering record to the high average impact velocity of ~11.5 km s−1 on Pallas—compared with an average of ~5.8 km s−1 for the asteroid belt—induced by Pallas’s high orbital inclination (i = 34.8°) and orbital eccentricity (e = 0.23). Compositionally, Pallas’s derived bulk density of 2.89 ± 0.08 g cm−3 (1σ uncertainty) is fully compatible with a CM chondrite-like body, as suggested by its spectral reflectance in the 3 μm wavelength region6. A bright spot observed on its surface may indicate an enrichment in salts during an early phase of aqueous alteration, compatible with Pallas’s relatively high albedo of 12–17% (refs. 7,8), although alternative origins are conceivable. High-resolution observations of the third largest asteroid, (2) Pallas, from SPHERE unveil a heavily cratered surface, probably due to Pallas’s inclined and eccentric orbit, a density almost equal to carbonaceous chondrites and hint at surficial salt-enriched spots.

An assessment of sanidine from the Fire Clay tonstein as a Carboniferous 40Ar/39Ar monitor standard and for inter-method comparison to U-Pb zircon geochronology

Radioisotopic geochronology applied to the high-resolution calibration of Earth history requires a set of synthetic and natural reference materials for both 40Ar/39Ar and U-Pb techniques that permit both inter-laboratory and inter-technique comparisons. The sanidine- and zircon-bearing Carboniferous Fire Clay tonstein provides a potential natural Paleozoic reference for these two widely used radioisotopic systems. Here we report results for both radioisotopic systems, examining the suitability of this tonstein as a geochronologic reference. Sanidine crystals from the Fire Clay and co-irradiated monitors from eight irradiation positions were divided into eleven 40Ar/39Ar experiments. Single-grain sanidine 40Ar/39Ar analyses (n = 263) of the simplest 9 experiments have internal 2σ uncertainties at the ±1 Myr level (±0.3%), with a range of dates between ~315 and ~317 Ma (~1% precision), similar to the observed dispersion in the Fish Canyon sanidine monitor dates. Forty-one U-Pb analyses by the CA-ID-TIMS method on carefully selected single Fire Clay tonstein zircons have produced 206Pb/238U dates with an average 2σ precision of ±0.23 Myr (0.14%). Our Fire Clay preferred mean 40Ar/39Ar date of 315.36 ± 0.10 Ma (±1.10 Ma: fully propagated 2σ uncertainty, relative to a Fish Canyon age of 28.201 Ma) is consistent with our weighted mean 206Pb/238U zircon date of 314.629 ± 0.039 Ma (±0.35 Ma: fully propagated 2σ uncertainty; n = 27). The good single-crystal reproducibility of the sanidine data and the overall consistency between the two chronometers suggest that the tonstein holds promise as a Paleozoic age reference material.

Mind the Gap: The Location of the Lower Edge of the Pair-instability Supernova Black Hole Mass Gap

Abstract Detections of gravitational waves are now starting to probe the mass distribution of stellar mass black holes (BHs). Robust predictions from stellar models are needed to interpret these. Theory predicts the existence of a gap in the BH mass distribution because of pair-instability supernovae. The maximum BH mass below the gap is the result of pulsational mass loss. We evolve massive helium stars through their late hydrodynamical phases of evolution using the open-source MESA stellar evolution code. We find that the location of the lower edge of the mass gap at 45 is remarkably robust against variations in the metallicity (≈3 ), the treatment of internal mixing (≈1 ), and stellar wind mass loss (≈4 ), making it the most robust predictor for the final stages of the evolution of massive stars. The reason is that the onset of the instability is dictated by the near-final core mass, which in turn sets the resulting BH mass. However, varying the reaction rate within its 1σ uncertainties shifts the location of the gap between 40 and 56 . We provide updated analytic fits for population synthesis simulations. Our results imply that the detection of merging BHs can provide constraints on nuclear astrophysics. Furthermore, the robustness against metallicity suggests that there is a universal maximum for the location of the lower edge of the gap, which is insensitive to the formation environment and redshift for first-generation BHs. This is promising for the possibility to use the location of the gap as a “standard siren” across the universe.

Middle to Late Miocene Age for the End of Amphibolite‐Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

AbstractWe report five new Rb‐Sr muscovite‐based isochron ages, which are the first to constrain the timing of amphibolite‐facies mylonitization of the Alpine Schist in the Southern Alps of New Zealand. The ages range from 13.1 ± 4.3 to 8.9 ± 3.2 Ma (2σ uncertainties) for mylonite directly above the Alpine Fault. The weighted mean age of 10.74 ± 0.57 Ma is within uncertainty of a published 40Ar/39Ar illite/mica upper‐intercept age of 11.5 ± 0.5 Ma measured at the same locality. The end of amphibolite‐facies mylonitization occurred at metamorphic conditions of ~560–570 °C and ~0.9–1.1 GPa as derived from pseudosection analysis in the NCTiKFMASH system. We interpret Miocene metamorphism to reflect transpressional crustal thickening and formation of a thick crustal root supporting early Southern Alps topography at or prior to 10.74 ± 0.57 Ma. Additional ~2–1 Ma Rb‐Sr biotite, 40Ar/39Ar muscovite, and 40Ar/39Ar biotite ages reflect isotopic closure during rapid cooling along the Alpine Fault in the Pleistocene. The Miocene mylonitization ages and the Pleistocene cooling ages define a distinct two‐stage cooling and exhumation history for the Alpine Schist with initial cooling of ~10 °C/Myr and exhumation rates of 2–4 km/Myr. Final cooling since ~2 Ma was >100 °C/Myr at exhumation rates of ~5–6 km/Myr. We interpret the two‐phase cooling history by movement of the mylonite through a strongly nonlinear thermal structure. An older 60.5 ± 0.7 Ma metamorphic event is also preserved as a Rb‐Sr crystallization age of a predeformational muscovite‐plagioclase assemblage in a sheared pegmatite.

Three wood isotopic reference materials for δ2H and δ13C measurements of plant methoxy groups

Methoxy groups (OCH3) of plants show specific stable carbon and hydrogen isotope patterns that are used for applications in biogeochemical, atmospheric, paleoclimatic and food research. The method of choice for determining stable hydrogen and carbon isotope values of methoxy groups (δ2HOCH3 and δ13COCH3 values) is the conversion to gaseous iodomethane (CH3I) and subsequent measurement by stable isotope ratio mass spectrometry. However, comparative measurements particularly for stable hydrogen isotopes of plant methoxy groups are limited due to the lack of suitable reference materials. We have prepared three batches of powdered wood samples (birch HUBG3, beech HUBG4, and tineo HUBG5) collected from different geographical locations to serve as long-term reference materials for normalization of δ2HOCH3 but also δ13COCH3 values. Methoxy contents of the three wood samples range between 4.7 and 5.4%. Methoxy groups from subsamples of the three homogenized wood samples were quantitatively converted to CH3I and δ2HOCH3 and δ13COCH3 values of this CH3I were calibrated against international reference substances by high-temperature conversion- and elemental analyzer isotope ratio mass spectrometry. The δ2HOCH and δ13COCH3 values of HUBG3 and HUBG4 at 1σ uncertainty calibrated to the VSMOW and VPDB isotopic δ-scale, respectively are −272.9 ± 1.5 mUr and −29.40 ± 0.13 mUr; and −239.1 ± 1.4 mUr and −30.17 ± 0.13 mUr, respectively. In addition, the calibrated δ2HOCH value of HUBG5 is −191.7 ± 0.8 mUr. Whilst the δ2HOCH3 values of the three wood samples span a relatively wide range (~−280 to −190 mUr) suitable for normalization of δ2HOCH3 values of most plant samples that have been reported so far, δ13COCH3 values are restricted to a composition (~−30 mUr) typical of wood methoxy groups. We suggest that the three investigated wood materials HUBG3–5 are ideally suited for long-term usage, inter-laboratory comparison, and together with the two recently reported methyl sulfate salts (HUBG1 and HUBG2) complete a new set of solid methoxy reference materials that cover almost the full range of plant methoxy groups reported so far. Therefore, we recommend replacing liquid CH3I and instead use the solid reference materials set HUBG1–5 for normalizing δ2HOCH3 and δ13COCH3 values to the respective δ-scales.

Uncertainty in strain-rate from field measurements of the geometry, rates and kinematics of active normal faults: Implications for seismic hazard assessment

Multiple measurements of the geometry, kinematics and rates of slip across the Auletta fault (Campania, Italy) are presented, and we use these to determine: (1) the spatial resolution of field measurements needed to accurately calculate a representative strain-rate; (2) what aspects of the geometry and kinematics would introduce uncertainty with regard to the strain-rate if not measured in the field. We find that the magnitude of the post last-glacial maximum throw across the fault varies along strike. If such variations are unnoticed, different values for a representative strain-rate, hence different results in seismic hazard calculations, would be produced. To demonstrate this, we progressively degrade our dataset, calculating the implied strain-rate at each step. Excluding measurements can alter strain-rate results beyond 1σ uncertainty, thus we urge caution when using only one measurement of slip-rate for calculating hazard. We investigate the effect of approximating the throw profile along the fault with boxcar and triangular distributions and show that this can underestimate or overestimate the strain-rate, with results in the range of 72–237% of our most detailed strain-rate calculation. We discuss how improved understanding of the potential implied errors in strain-rate calculations from field structural data should be implemented in seismic hazard calculations.

Using machine learning to study the kinematics of cold gas in galaxies

Abstract Next generation interferometers, such as the Square Kilometre Array, are set to obtain vast quantities of information about the kinematics of cold gas in galaxies. Given the volume of data produced by such facilities astronomers will need fast, reliable, tools to informatively filter and classify incoming data in real time. In this paper, we use machine learning techniques with a hydrodynamical simulation training set to predict the kinematic behaviour of cold gas in galaxies and test these models on both simulated and real interferometric data. Using the power of a convolutional autoencoder we embed kinematic features, unattainable by the human eye or standard tools, into a three-dimensional space and discriminate between disturbed and regularly rotating cold gas structures. Our simple binary classifier predicts the circularity of noiseless, simulated, galaxies with a recall of $85\%$ and performs as expected on observational CO and HI velocity maps, with a heuristic accuracy of $95\%$. The model output exhibits predictable behaviour when varying the level of noise added to the input data and we are able to explain the roles of all dimensions of our mapped space. Our models also allow fast predictions of input galaxies’ position angles with a 1σ uncertainty range of ±17○ to ±23○ (for galaxies with inclinations of 82.5○ to 32.5○, respectively), which may be useful for initial parameterisation in kinematic modelling samplers. Machine learning models, such as the one outlined in this paper, may be adapted for SKA science usage in the near future.

Cosmological constrains on minimally and non-minimally coupled scalar field models

Abstract We study the minimally and non-minimally coupled scalar field models as possible alternatives for dark energy, the mysterious energy component that is driving the accelerated expansion of the universe. After discussing the dynamics at both the background and perturbation level, we confront the two models with the latest cosmological data. After obtaining updated constraints on their parameters we perform model selection using the basic information criteria. We found that the ΛCDM model is strongly favored when the local determination of the Hubble constant is not considered and that this statement is weakened once local H0 is included in the analysis. We calculate the parameter combination $S_8=\sigma _8\sqrt{\Omega _{m}/0.3}$ and show the decrement of the tension with respect to the Planck results in the case of minimally and non-minimally coupled scalar field models. Finally, for the coupling constant between DE and gravity, we obtain the constraint $\xi \simeq -0.06^{+0.19}_{-0.19}$, approaching the one from solar system tests |ξ| ≲ 10−2 and comparable to the conformal value ξ = 1/6 at 1σ uncertainty.

Shock Tube Measurement of the CH3 + C2H6 → CH4 + C2H5 Rate Constant.

The rate constant for the CH3 + C2H6 → CH4 + C2H5 reaction was studied behind reflected shock waves at temperatures between 1369 and 1626 K and pressures from 8.6 to 47.4 atm in mixtures of methane, ethane, and argon. Ethylene time histories were measured using laser absorption of radiation from a carbon dioxide gas laser near 10.532 μm. The resulting rate constant data can be represented by the Arrhenius equation k (T) = 3.90 × 1013 exp(-16670 cal/mol/RT) cm3 mol-1 s-1. We believe this is the first study to extend experimental data for this rate constant to temperatures above 1400 K. The overall 2σ uncertainty of the current data is +18%/-21% resulting primarily from uncertainties associated with the influence of secondary reactions and the fitting of rapidly changing species time histories at the higher temperatures.

Exploring the law of detrital zircon: LA-ICP-MS and CA-TIMS geochronology of Jurassic forearc strata, Cook Inlet, Alaska, USA

AbstractUranium-lead (U-Pb) geochronology studies commonly employ the law of detrital zircon: A sedimentary rock cannot be older than its youngest zircon. This premise permits maximum depositional ages (MDAs) to be applied in chronostratigraphy, but geochronologic dates are complicated by uncertainty. We conducted laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) of detrital zircon in forearc strata of southern Alaska (USA) to assess the accuracy of several MDA approaches. Six samples from Middle–Upper Jurassic units are generally replete with youthful zircon and underwent three rounds of analysis: (1) LA-ICP-MS of ∼115 grains, with one date per zircon; (2) LA-ICP-MS of the ∼15 youngest grains identified in round 1, acquiring two additional dates per zircon; and (3) CA-TIMS of the ∼5 youngest grains identified by LA-ICP-MS. The youngest single-grain LA-ICP-MS dates are all younger than—and rarely overlap at 2σ uncertainty with—the CA-TIMS MDAs. The youngest kernel density estimation modes are typically several million years older than the CA-TIMS MDAs. Weighted means of round 1 dates that define the youngest statistical populations yield the best coincidence with CA-TIMS MDAs. CA-TIMS dating of the youngest zircon identified by LA-ICP-MS is indispensable for critical MDA applications, eliminating laser-induced matrix effects, mitigating and evaluating Pb loss, and resolving complexities of interpreting lower-precision, normally distributed LA-ICP-MS dates. Finally, numerous CA-TIMS MDAs in this study are younger than Bathonian(?)–Callovian and Oxfordian faunal correlations suggest, highlighting the need for additional radioisotopic constraints—including CA-TIMS MDAs—for the Middle–Late Jurassic geologic time scale.

The SPICE project: Production rates of cosmogenic 21Ne, 10Be, and 14C in quartz from the 72 ka SP basalt flow, Arizona, USAu

The SP lava flow is a quartz-, olivine- and pyroxene-bearing basalt with an 40 Ar/ 39 Ar age of 72 ± 4 ka (2σ). The flow is preserved in the desert climate of northern Arizona, USA. Its unweathered appearance and the lack of soil development indicate it has undergone negligible erosion and/or burial, making it an ideal site for direct calibration of cosmogenic nuclide production rates. Cross-calibrated production rates and production rate ratios for cosmogenic 21 Ne, 10 Be, and 14 C have been determined from SP flow quartz. Production rate ratios for 21 Ne/ 10 Be, 21 Ne/ 14 C, and 14 C/ 10 Be are based on the total, local production rates of each cosmogenic nuclide, independent of scaling models, and have error-weighted means (±2σ uncertainty) of 4.44 ± 0.32, 1.43 ± 0.10, and 2.85 ± 0.21, respectively. Error-weighted mean, sea-level, high latitude (SLHL) total reference production rates of 21 Ne, 10 Be, and 14 C are 17.0 ± 1.1, 3.84 ± 0.27, and 11.2 ± 0.6 at/g/yr (2σ), respectively, using time-independent Lal (1991)/Stone (2000) ( St ) scaling factors. St scaled spallogenic 10 Be and 14 C rates are 3.73 ± 0.26 and 9.2 ± 0.6 at/g/yr, respectively. 21 Ne and 10 Be production rates are integrated over the past 72 ka, whereas 14 C production rates are integrated over 25 ka, the time at which SP flow quartz has reached saturation with respect to 14 C. These rates overlap within 2σ uncertainty with other St -scaled production rates in the literature, including the total reference SLHL 21 Ne production rate of Niedermann (2000), which is revised in this paper to 16.8 ± 3.3 at/g/yr (2σ; St scaling) to reflect a recent change in age control at the Sierra Nevada sites. All SLHL production rates are lower if time- dependent Sf , Sa , and Lm scaling factors are used. For example, error-weighted mean, sea-level, high latitude (SLHL) total reference production rates for 10 Be as calculated in the CREp online calculator range from 3.49 ± 0.23 to 3.74 ± 0.25 at/g/yr (2σ), using time-dependent Lm scaling factors. Commonly used SLHL 10 Be and 14 C production rates in the literature were calibrated on surfaces that have been exposed to cosmic rays for less than 20 ka. Between 20 and 50 ka, the geomagnetic field is proposed to have been weaker than it is today. Production rates of cosmogenic nuclides increase during periods of weaker geomagnetic field strength. However, our study finds no measureable difference between St -scaled production rates of cosmogenic 21 Ne and 10 Be over the past 20 ka and St -scaled 21 Ne and 10 Be production rates over the past 72 ka. As such, the study suggests that 21 Ne and 10 Be production rates in quartz were not significantly greater during the proposed period of decreased magnetic strength from 20 to 50 ka. • Total reference production rates at SLHL are calculated in 72 ka quartz. • Cosmogenic 21 Ne, 10 Be, and 14 C rates ( St ): 17.0 ± 1.1, 3.84 ± 0.27, and 11.2 ± 0.6 at/g/yr. • These rates agree with St scaled production rates over past 20 ka in literature. • There is a proposed period of decreased geomagnetic field strength from 20 to 50 ka. • SPICE rates do not record increased cosmogenic nuclide production over past 72 ka.