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4. Bayesian integration of astrochronology and radioisotope geochronology

   https://doi.org/10.5194/gchron-6-107-2024  

   Trayler, Robin B; Meyers, Stephen R; Sageman, Bradley B; Schmitz, Mark D (January 2024, Geochronology)

   na (Ed.)

   Abstract. Relating stratigraphic position to numerical time using age–depth models plays an important role in determining the rate and timing of geologic and environmental change throughout Earth history. Astrochronology uses the geologic record of astronomically derived oscillations in the rock record to measure the passage of time and has proven to be a valuable technique for developing age–depth models with high stratigraphic and temporal resolution. However, in the absence of anchoring dates, many astrochronologies float in numerical time. Anchoring these chronologies relies on radioisotope geochronology (e.g., U–Pb, 40Ar/39Ar), which produces high-precision (<±1 %), stratigraphically distributed point estimates of age. In this study, we present a new R package, astroBayes, for a Bayesian inversion of astrochronology and radioisotopic geochronology to derive age–depth models. Integrating both data types allows reduction in uncertainties related to interpolation between dated horizons and the resolution of subtle changes in sedimentation rate, especially when compared to existing Bayesian models that use a stochastic random walk to approximate sedimentation variability. The astroBayes inversion also incorporates prior information about sedimentation rate, superposition, and the presence or absence of major hiatuses. The resulting age–depth models preserve both the spatial resolution of floating astrochronologies and the accuracy as well as precision of modern radioisotopic geochronology. We test the astroBayes method using two synthetic datasets designed to mimic real-world stratigraphic sections. Model uncertainties are predominantly controlled by the precision of the radioisotopic dates and are relatively constant with depth while being significantly reduced relative to “dates-only” random walk models. Since the resulting age–depth models leverage both astrochronology and radioisotopic geochronology in a single statistical framework they can resolve ambiguities between the two chronometers. Finally, we present a case study of the Bridge Creek Limestone Member of the Greenhorn Formation where we refine the age of the Cenomanian–Turonian boundary, showing the strength of this approach when applied to deep-time chronostratigraphic questions. 

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5. U–Pb ID-TIMS geochronology using ATONA amplifiers

   https://doi.org/10.1039/d0ja00135j  

   Szymanowski, Dawid; Schoene, Blair (June 2020, Journal of Analytical Atomic Spectrometry)

   We document the performance of new ATONA (‘aA to nA’) amplifiers installed on an Isotopx Phoenix thermal ionisation mass spectrometer (TIMS) at Princeton University and evaluate their suitability for high-precision analyses of Pb and U isotopes in pg- to ng-size samples characteristic for U–Pb geochronology. The new amplifiers are characterised by low and stable noise levels comparable to 10 12 to 10 13 ohm resistors, response time <0.5 s, exceptional gain stability <1 ppm and a vast dynamic range theoretically allowing to quantify signals from aA (10 −18 A) to nA (10 −9 A) level. We measured a set of Pb standards, synthetic U–Pb solutions and natural zircons at currents of 2 × 10 −16 to 2 × 10 −12 A (corresponding to intensities of 20 μV to 200 mV relative to a 10 11 ohm amplifier) to assess the utility of ATONA in replacing ion counting for the smallest samples. The results show a clear precision benefit of using ATONA-Faraday detection over Daly ion counting for ion currents of >10 −14 A (1 mV relative to a 10 11 ohm amplifier or ca. 60 kcps). As such currents are routinely achievable for major Pb peaks of interest ( 205–208 Pb) in natural samples containing more than ca. 10 pg Pb* (radiogenic Pb), we expect ATONA-Faraday detection to find broad applications in U–Pb geochronology. Its practical use for low-blank, radiogenic samples continues to require ion counting for 204 Pb, either with a fixed Faraday–ion counter gain or using a dynamic two-step ( e.g. FaraDaly) method. Routine adoption of ATONA-Faraday collection in place of ion counting for most major Pb and U isotopes has the potential to increase sample throughput and precision, both improving the accessibility of isotope dilution (ID)-TIMS geochronology and pushing this technique towards better reproducibility. 

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