Toward a composite orbital chronology for the Late Cretaceous and Early Palaeocene GPTS

Drill sites recovered from the South Atlantic by the Deep Sea Drilling Proj ect provide excellent magnetostratigraphies for the time-interval of polari ty chrons C33r through C28n (83-62.5 Ma: Cande & Kent time-scale). Cyclic p atterns of carbonate sedimentation, with a mean repeat time of ca. 20 ka, c an be correlated between sites. The cycles show the full hierarchy of eccen tricity amplitude modulations that would be expected of precessional orbita l forcing. A significant modulation component exists at ca. 400 ka, which i n all likelihood matches the modern 404 ka repeat time for eccentricity. Th is modulation pattern may provide the 'tuning fork' for tying cyclical sedi mentation to the most stable astronomical eccentricity period. A ca. 2.5 Ma modulation also emerges that seems surprisingly similar to the modern long -wavelength eccentricity modulation of precessional amplitude, which should average 2.425 Ma. Results of 'cyclochronology' of polarity chrons C29n-C31 n suggest that the Cande & Kent time-scale will need to be revised to allow for a more gentle change in South Atlantic sea-floor spreading than modell ed by those authors.