The quest for refined calibration of the Jurassic time-scale

The Jurassic time-scale assigns numerical ages to boundaries of chronostratigraphic units. A well-established ammonite biochronology forms the basis of stage definitions that are being formalized by Global Stratotype Sections and Points. Two major updates of the Jurassic time-scale (referred to as JTS2000 and GTS2004) were published recently. JTS2000 relies heavily on U-Pb and 40Ar/39Ar dates, whereas GTS2004 emphasizes complementary scaling methods using strontium (Sr) isotope stratigraphy, cyclostratigraphy and magnetostratigraphy. U-Pb and 40Ar/39Ar dates remain the backbone of the time-scale; relevant new developments are reviewed briefly. Fourteen recently published ages are added to the database of calibration points. Floating cyclostratigraphies already cover a significant portion of the Jurassic, allowing measurements of durations that need to be anchored and linked to chronostratigraphy. Where tie-points are sparse, reliance on scaling methods remains necessary. Sr isotope stratigraphy and magnetostratigraphy are increasingly sophisticated and useful for both correlation and scaling. Further refinements of calibration are expected from more accurate and densely spaced radioisotopic age tie-points, especially in the Late Jurassic, and from an extended coverage of Jurassic astrochronology. In the computer era, time-scales increasingly are being delivered digitally, updated continuously and accessed interactively by their users.