Abstract
Over the past decade the integration of astrochronology and U/Pb thermal ionization mass spectrometry dating has resulted in major improvements in the Devonian time scale, which allowed for accurate determination of ages and rates of change in this critical interval of Earth history. However, widely different durations have been published for the Middle Devonian Eifelian stage. Here we aim to solve this discrepancy by building an astronomically calibrated time scale using a high-resolution geochemical data set collected in the early to late Eifelian outer-ramp and deep-shelf deposits of the Seneca section (Appalachian Basin, Western New York, USA). The Middle Devonian Eifelian Stage (GTS2012; base at 393.3 ± 1.2 m.y. and duration estimate of 5.6 ± 1.9 m.y.), is bracketed by two major bioevents, respectively the Choteč event at its base and the Kačák event just prior to the Eifelian–Givetian boundary. To capture the record of Milankovitch-scale climatic cycles and to develop a model of the climatic and oceanographic variations that affected the Appalachian Basin during the Eifelian, 750 samples were collected at typically 2.5 cm intervals across the Seneca section. Major and trace elements were measured on each sample with an inductively coupled plasma–optical emission spectrometer. To estimate the duration of the Seneca section sampled, we applied multiple spectral techniques such as harmonic analysis, the multi-taper, and evolutionary spectral analysis, and we tuned the Log10Ti series using the short orbital eccentricity ∼100 k.y. cycle. Then, to assess the reliability of our cyclostratigraphic interpretation we ran the Average Spectral Misfit method on selected proxies for detrital input variation. The estimated duration derived using this method falls in the range of durations estimated with the tuning method. Using the approximate position of the Emsian–Eifelian and Eifelian–Givetian boundaries, constrained within <1 m, the proposed estimation of the total duration of the Eifelian age is ∼5 m.y. Interpolated from the high-resolution U-Pb radiometric age available for the Tioga F Bentonite, the numerical ages of the Emsian–Eifelian and the Eifelian–Givetian were respectively recalibrated at 393.39 Ma and 388.24 Ma. The uncertainty from the radiometric date is respectively ± 0.86 Ma and ± 0.86 Ma.
Optimal Octagonal Hooked Collar Countermeasure to Reduce Scour Around a Single Bridge Pier
