A time like our own? Radioisotopic calibration of the Ordovician greenhouse to icehouse transition

摘要

Tiered interpolation, a new timescale methodology, was used to construct the first radioisotopically-calibrated composite δ ~(13)C curve for the Ordovician period using sanidine ~(40)Ar/ ~(39)Ar age determinations and existing U-Pb geochronology and biostratigraphic zonation. Tiered interpolation intercalates and temporally scales the numerical age of lithostratigraphic horizons by conducting a series of nested projections between hierarchical temporal control points. For primary control points, new ~(40)Ar/ ~(39)Ar ages and legacy U-Pb geochronology were screened to avoid analyses affected by inheritance and daughter loss and calibrated to reflect modern decay constants and standard values. Ages for secondary, tertiary, etc. control points are obtained via linear interpolation of between higher order control points. In scaling the Ordovician δ ~(13)C composite, the following control point order was applied: (1) radioisotopic ages (2) graptolite Zones, (3) index taxa-based on speciation events (North Atlantic conodont Zones), (4) North American Mid-continent conodont zones, and (5) stratal thicknesses at δ ~(13)C sampled sections. The resulting timescale utilizes the highest resolution of each component, is internally consistent, and is re-scalable as more precise radioisotopic ages become available. It provides a robust framework for independently assessing the accuracy of biostratigraphic composite timescales because it does not rely an assumption of quasi-continuous sediment accumulation and/or speciation. To better calibrate the Late Ordovician and resolve a discrepancy between U-Pb and ~(40)Ar/ ~(39)Ar ages, three new ~(40)Ar/ ~(39)Ar ages were determined via the laser fusion of multiple single sanidine phenocrysts from three bentonitic ash beds from the Late Ordovician marine strata of the upper Mississippi valley where the record of Taconic volcanism is most complete. Fusions of 275 individual sanidine crystals 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±1.4Ma (51 of 57), 450.7±1.4Ma (39 of 74), and 450.3±1.9Ma (96 of 144) for the Millbrig, Dygerts, and Rifle Hill bentonites, respectively (2σ analytical uncertainties relative to 28.201Ma for FCs). The Millbrig age is consistent with the existing U-Pb ages for both the underlying Deicke bentonite and the Kinnekulle bentonite of Sweden. The new age model permits the assembly of the first complete radioisotopically-calibrated composite δ ~(13)C curve for the Ordovician, the first icehouse to occur subsequent to the Cambrian explosion. The resulting δ ~(13)C composite integrates all available graptolite and conodont biostratigraphic with radioisotopic ages and indicates that previous biostratigraphic composites incorporate 2σ errors up to ~5Ma. When viewed without temporal distortions, isotopic carbon excursions (ICEs) in the Ordovician appear to have occurred at a similar tempo as ICEs in the better resolved Cenozoic greenhouse to icehouse transition. Although boundary conditions for oceanography, biogeography, and continental configuration are strikingly different, the tempo of isotopic changes, growth of south-polar ice sheets, and concurrent oceanic and geomorphic responses bears both similarities and differences with the better understood Cenozoic era.