High-precision U–Pb zircon age constraints on the Carboniferous–Permian boundary in the southern Urals stratotype Jahandar Ramezani a , Mark D. Schmitz b, ⁎ , Vladimir I. Davydov b , Samuel A. Bowring a , Walter S. Snyder b , Clyde J. Northrup b a Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA b Department of Geosciences, Boise State University, Boise, Idaho 83702, USA Received 25 July 2006; received in revised form 27 January 2007; accepted 29 January 2007 Edited by: R.W. Carlson Available online 3 February 2007 Abstract Volcanic-ash beds within fossiliferous strata of the southern Uralian foredeep of Russia have been dated by the high-precision ID-TIMS U–Pb zircon method, to constrain the age of the Carboniferous–Permian transition. The stratigraphic section exposed at the Usolka locality, an auxiliary stratotype for the Carboniferous–Permian boundary, contains a detailed multi-taxa biostratigraphy with a well-resolved boundary succession tied to the Global Stratotype Section and Point (GSSP) at Aidaralash Creek (Kazakhstan). Four tuffs bracketing the Carboniferous–Permian transition in the Usolka section closely constrain the age of the boundary to 298.90 + 0.31/- 0.15 Ma (2σ), including both analytical and stratigraphic uncertainty. These data substantially improve the calibration of the geologic time scale in the Late Paleozoic, and presage the radiometric age resolution possible for the late Carboniferous and early Permian stages through continued analysis of ash beds in the southern Urals stratotypes. By stabilizing the numeric age for the base of the Permian, these data also provide an important link between the timing of Permo-Carboniferous glaciation in Gondwana and its far-field climatic and sedimentological consequences in Euramerica. © 2007 Elsevier B.V. All rights reserved. Keywords: Carboniferous; Permian; boundary; U–Pb; Zircon; Ural Mountains 1. Introduction The geologic time scale is a fundamental tool in Earth sciences research — the global correlation of events and the calculation of rates of geological and biological pro- cesses during Earth's history depend directly on the accuracy and precision of the geologic time scale. Yet despite its critical role, the numerical calibration of sig- nificant intervals of the time scale remains poor due to a lack of accurate, high-precision radiometric ages tied to high-resolution biostratigraphic contexts. The Late Pa- leozoic provides an important example: commonly cited time scales differ by as much as 10 Ma in the estimated age of the Carboniferous–Permian (C–P) boundary, and vary by as much as 500% in the inferred duration of various stages [1–5]. Without a precisely calibrated Pennsylvanian and Permian time scale, basic questions regarding the final assembly and early evolution of Pangaea, the paleoclimatic transition from the paleocli- matic transition from Pennsylvanian ‘icehouse’ to the Permian ‘hothouse’ conditions, and a host of other late Paleozoic problems will remain unresolved [6–11]. Earth and Planetary Science Letters 256 (2007) 244 – 257 www.elsevier.com/locate/epsl ⁎ Corresponding author. Tel.: +1 208 426 5907; fax: +1 208 426 4061. E-mail address: markschmitz@boisestate.edu (M.D. Schmitz). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.01.032