CYCLOSTRATIGRAPHIC IMPLICATIONS OF DEVONIAN-CLIMATE ASTRONOMICAL FORCING

A Devonian magnetic susceptibility (MS) record was obtained from Uppermost-Eifelian to Late-Frasnian limestones along the southern border of the Dinant Synclinorium in Belgium. In these carbonate ramp and platform deposits, large-scale variations (tens to hundreds of meters) in the MS signal are ascribed to sea-level fluctuations, while faster variations (several meters) are interpreted as changes in the flux of magnetic minerals towards the marine system, controlled by precipitation intensity. Spectral analysis highlights persistent cycles in the MS signal. These cycles are interpreted as astronomical cycles by involving both chrono- and biostratigraphic information, complemented by theoretical knowledge of sedimentation rates in different depositional environments. The different time constraints indicate that some of the determined sedimentary cycles correspond to precession (~17ka) cycles, while others agree with obliquity (~33ka) cycles. To correctly understand these astronomically forced cycles in the MS signal from a climatologic and oceanographic point of view, a hypothesis is proposed that explains, in a simplified way, how precession and obliquity can account for a southward shift of the Intertropical Convergence Zone (ITCZ), and for the consequent increase in precipitation over the palaeolocation of Belgium. For the “La Couvinoise” section (Uppermost-Eifelian to Lower-Givetian; 85 meters thick), the well-developed precessional cycles, together with the accepted isotopic age of the Eifelian/Givetian boundary (388.1 ± 2.6 Ma), lead to the construction of a “Floating Point time scale". This time scale allows, for every point in the “La Couvinoise” section, the determination of its relative age compared to the Eifelian/Givetian boundary with an accuracy of less than 10,000 years. These results indicate that also in the Paleozoic, there is potential for the geological time scale to undergo important improvements through orbital tuning.