Floating astronomical time

Lower Cretaceous (Albian) deep-sea sediments (coccolith/globigerinacean marls) of the Umbria-Marche Apennines were drilled in 1979 near Piobbico, Italy. The drillcore revealed sustained, rhythmic sedimentary bedding, with drab facies representing normal stratified conditions, and red facies indicating downwelling warm saline waters. The drab facies is overprinted by cm- to dm-scale redox oscillations; these oscillations occur with a strong 5:1 bundling pattern. A digitized photographic log of the core was greyscale-scanned and analyzed for orbital frequencies. Strong eccentricity-modulated precession cycles occur through most of the core; an obliquity signal gains prominence in dominantly black shale facies. The bundling patterns allow the interpretation of 406-ky eccentricity cycles (see yellow row above), yielding a duration of 11.8 ± 0.4 Ma for the Albian Stage (Grippo et al., 2004). This chronological estimate was used as a 'floating' ATS in GTS2004 (Gradstein et al., 2004).

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