Magnetic records of climatic cycles from mid-Cretaceous hemipelagic sediments of the Vocontian Basin, SE France

A study of mid-Cretaceous hemipelagic sediments from the Vocontian Basin in SE France has been carried out in order to identify palaeoclimatic cycles driven by changes in the Earth's orbit ('Milankovitch cycles'). The calibration of cyclic behaviour of magnetic properties can provide sedimentary chronology on a finer timescale than biostratigraphy or magnetostratigraphy. The observed Aptian succession of marls and claystones with intercalated black shales and marly limestones shows a cyclic pattern of variation in magnetic susceptibility. Constant values of isothermal remanent magnetisation ratios (S-ratios) indicate a constant composition of ferrimagnetic minerals and thus continuous sedimentation without elastic components delivered from secondary sources. The curves of isothermal remanent magnetisation (M-ir), which is sensitive to ferromagnetic minerals, correspond only in part to the susceptibility, so that the susceptibility signal is mainly dominated by fluctuations of the paramagnetic mineral content. While the wave length of susceptibility variations expresses the precessional and eccentric cycles, a second precessional cycle is identified in the frequency spectrum of the M-ir. Our results demonstrate the suitability of rock magnetic properties for identifying and interpreting climatic cycles in these Mesozoic successions. (C) 2001 Academic Press.