Rhenium-Osmium (Re-Os) dating of black shale of the proposed Global Stratotype Section and Point (GSSP) for the Oxfordian-Kimmeridgian boundary, at Staffin Bay, Isle of Skye, U.K., yields an age of 154.1 ± 2.2 Ma. This Re-Os age presents a 45 % (1.8 m.y.) improvement in precision for the basal Kimmeridgian. It demonstrates that the duration of the Kimmeridgian is nominally 3.3 m.y. and thus is 1.6 m.y. shorter than previously indicated. The new Re-Os age together with a recently determined 40Ar-39Ar age for the basal Berriasian suggest that the duration of the Kimmeridgian and Tithonian were nominally ~650 k.y. shorter than previously determined. The Re-Os data for the Staffin black shale indicates that the seawater Os isotope composition at the Oxfordian/Kimmeridgian boundary was radiogenic (187Os/188Os = 0.53), similar to that for the Early Tithonian, but much higher than the Late Callovian (187Os/188Os = 0.27). These Os isotope compositions suggest that the rate of continental weathering had increased from the Callovian to Kimmeridgian/Tithonian.
David Selby: Department of Earth Sciences, Science Laboratories, University of Durham, Durham, DH1 3LE, UK,