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 × 10 4 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.