TIERED INTERPOLATION OF RADIOISOTOPIC AND BIOSTRATIGRAPHIC GEOCHRONOLOGY RESOLVES THE TEMPO OF THE ORDOVICIAN ICEHOUSE TO GREENHOUSE TRANSITION

We introduce here a new numerical timescale methodology that permits construction of the first radioisotopically-calibrated composite δ¹³C curve for the Ordovician period. Tiered interpolation is a database intercalation methodology that temporally scales lithostratigraphic horizons between a series of hierchically-nested temporal control points. Numerical ages for stratal horizons or stacked series of isotopically-sampled horizons are calculated via linear interpolation between adjacent control points. Tiered interpolation timescale utilizes the highest resolution of each time-resolving component, is re-scalable as more precise radioisotopic ages become available, and provides a robust framework for propagating full age uncertainties.

For the Ordovician δ¹³C composite, both sanidine ⁴⁰Ar/³⁹Ar and zircon U-Pb ages are used as primary control points. Radioisotopic ages were screened to limit inaccuracy arising from daughter-loss and inheritance, and have been calibrated to account for new decay constant and standard measurements. Between primary control points (radioisotopic ages), secondary (graptolite biostratigraphy), tertiary (speciation event-based conodont biostratigraphy), quaternary (North American midcontinent conodont zones), and quinary (lithostratigraphy) control points are used to distribute time within the overall composite.

To supplement the geochronology of the Late Ordovician, three ⁴⁰Ar/³⁹Ar ages were determined via the laser fusion of single sanidine phenocrysts from three Late Ordovician bentonitic ash beds from the upper Mississippi valley (UVM). Fusions of 275 individual sanidine from the Millbrig, Dygerts, and Rifle Hill bentonites yield largely Gaussian apparent age distributions with a small number of readily-identified outliers and stratigraphically-consistent weighted mean ages of 454.1 ± 1.9 Ma, 450.7 ± 1.9 Ma, and 450.3 ± 2.2 Ma for the Millbrig, Dygerts, and Rifle Hill bentonites, respectively (fully propagated 2σ uncertainties relative to 28.201 Ma for FCs). The resulting δ¹³C composite reveals a similar tempo for isotopic carbon excursions (ICEs) that accompanied the growth ice sheets in the southern hemisphere during both the Ordovician and the better-resolved Cenozoic.