Search for: All records

Isotopic analysis of phosphate oxygen from bones and teeth (18Op/16Op, δ18Op) is a common tool used to investigate modern and ancient ecosystems and their climate. However, existing methods have expanded to use pretreatments for organic removal, require large sample sizes, or require extended precipitation timing. All together, these factors could affect accuracy and precision of δ18Op measurement by promoting the formation of oxygen-bearing or nitrogen-rich contaminants. However, the nature and occurrence of contamination are not fully explored. Here we sought to develop a method of silver phosphate precipitation that tests the effect of different sample treatments and reduced sample sizes while preserving sample isotopic composition. Our protocol (SPORA) precipitates Ag3PO4 crystals from ∼1.5 mg of starting material while purifying phosphate from contaminants, like nitrogen or carbonate. Isolation and purification of phosphate are achieved with an anion exchange resin, followed by precipitation of silver phosphate using an updated silver ammine solution that targets small amounts of phosphate in solution. We used a variety of phosphate oxygen reference materials and biogenic apatite materials, such as modern and fossil specimens with varying collagen content, to test the SPORA protocol and its effects on the resultant phosphate oxygen isotopic composition. Results were then compared to those from another published silver phosphate precipitation method (i.e., Rapid University of Chicago Dilute, Rapid UC). Overall, δ18Op values of standards and biogenic apatites were similar between protocols (R2 = 0.99, p << 0.05). In addition to isotope composition comparisons, UV–Vis spectroscopy and Fourier Transform Infrared (FTIR) analyses discerned phosphate recovery and material composition of crystals precipitated via different protocols, respectively. We found that the resin i) may retain ∼10% of phosphate with no isotopic effects and ii) the SPORA protocol produces Ag3PO4 with more accurate δ18Op measurements by preventing the formation of contaminant oxygen phases, silver oxide (Ag2O) and silver carbonate (Ag2CO3), that confound the phosphate oxygen isotope composition. The SPORA Ag3PO4 precipitation procedure overcomes analytical limitations such as sample size and collagen contamination, conditions that other procedures for δ18Op analysis cannot address simultaneously. The SPORA protocol can be used on a large array of bioapatite materials for paleoecological, paleoclimatic, and archeological applications, while reducing the required sample size and ensuring pure Ag3PO4 for isotopic analysis. 

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. 

This releaser corresponds to publication of the manuscript 

Abstract The numerous biotic, climatic, and tectonic events of the Devonian cannot be correlated and investigated without a well-calibrated time scale. Here, we updated the calibration of the Devonian time scale using a Bayesian age-depth model that incorporates radioisotopic ages and astrochronology durations. We used existing radioisotopic ages collected and harmonized in the last two geologic time scale compilations, as well as new U-Pb zircon ages from Emsian {Hercules I K-bentonite, Wetteldorf, Germany: 394.290 ± 0.097(0.21)[0.47] Ma} and Eifelian K-bentonites {Tioga B and Tioga F K-bentonites, Fayette, New York, USA: 390.82 ± 0.18(0.26)[0.48] Ma and 390.14 ± 0.14(0.23)[0.47] Ma, respectively}. We anchored floating astrochronology stage durations on radioisotopic ages and chained astrochronologic constraints and uncertainty together to extrapolate conditioning age likelihoods up or down the geologic time scale, which is a new method for integrating astrochronology into age-depth modeling. The modeling results in similar ages and durations for Devonian stages regardless of starting biostratigraphic scaling assumptions. We produced a set of rescaled biostratigraphic zonations, and a new numerical calibration of Devonian stage boundary ages with robust uncertainty estimates, which allow us to evaluate future targets for Devonian time scale research. These methods are broadly applicable for time scale work and provide a template for an integrated stratigraphic approach to time scale modeling.