Re–Os depositional ages and seawater Os estimates for the Frasnian–Famennian boundary: Implications for weathering rates, land plant evolution, and extinction mechanisms

Four TOC-rich shale intervals spanning the Frasnian–Famennian (F–F) boundary were recovered in a drillcore (West Valley NX-1) from western New York (USA) and radiometrically dated using Re–Os. Two of the black shale intervals (WVC785 from ∼ 2.9 m below, and WVC754 from ∼ 6.4 m above the F–F boundary, respectively) yielded statistically overlapping ages with uncertainties of < 1.1%. An interpolated age and associated graphically determined uncertainty of 372.4 ± 3.8 Ma provides new absolute age constraints on the F–F boundary. This date is ∼ 4.1 Ma younger than the latest proposed F–F boundary age of 376.1 Ma obtained by interpolation of U–Pb dates from volcanic zircon [Kaufmann, B., 2006. Calibrating the Devonian Time Scale: A synthesis of U–Pb ID-TIMS ages and conodont stratigraphy. Earth-Science Reviews 76, 175–190], and within uncertainty of the International Commission on Stratigraphy accepted date of 374.5 ± 2.6 Ma. A third date (from sample WVC802, ∼ 8.2 m beneath the F–F boundary) yielded an imprecise age of 357 ± 23 Ma, owing in part to a limited Re/Os range. The initial 187Os/188Os (0.45 to 0.47), reflecting contemporaneous seawater Os values, are low but similar to the value of 0.42 reported for the Exshaw Fm (Canada) at the Devonian–Mississippian boundary (ca. 361 Ma) [Selby D., Creaser R.A., 2005. Direct radiometric dating of the Devonian–Mississippian time-scale boundary using the Re–Os black shale geochronometer. Geology 33, 545–548]. This may suggest fairly constant and low global continental weathering rates during the Late Devonian, although in view of the short residence time of Os in seawater (∼ 1–4 × 104 yr), further measurements are needed to assess potential short-term variation in seawater Os ratios. Owing to low Os and Re abundances at the F–F boundary, our data are inconsistent with long-term volcanism and bolide impact as potential Late Devonian mass extinction mechanisms. In addition, the Frasnian–Famennian ocean appears to have been depleted with respect to Re, possibly indicating an exhaustion of the Re seawater reservoir owing to high burial rates of redox-sensitive elements under dysoxic/anoxic conditions leading up to the F–F boundary.