Chapter Four - Mechanisms of Preservation of the Eccentricity and Longer-term Milankovitch Cycles in Detrital Supply and Carbonate Production in Hemipelagic Marl-Limestone Alternations
Résumé
Eccentricity cycles often exert a strong influence on sedimentary series, while they only have weak powers in the insolation series. Three case studies previously published in Early Cretaceous hemipelagic marl-limestone alternations of the Tethyan area are reviewed here to evaluate possible mechanisms of transfer of power from the precession to the eccentricity band. In all cases, the sedimentation rates vary cyclically, following the 405-kyr and the 2.4-myr eccentricity cycles. Maximums of sedimentation rates occur in more clayey intervals, suggesting a strong eccentricity forcing on detrital supply and consequently on sedimentation rates. In all sections, proxies related to purely detrital input show an overwhelming influence of the 405-kyr and the 2.4-myr eccentricity cycle compared to the other Milankovitch components. Proxies related to the carbonate production show lower amplitudes of the 405-kyr cycle and enhanced 100-kyr eccentricity cycle, which can dominate the power in the Milankovitch band.Memory effects of erosional and pedogenetic processes are proposed as powerful mechanisms of transfer of amplitude from the precession to the eccentricity band. Highly evolved pedogenesis under more humid climates have longer memory effects, which favor the 405-kyr and the 2.4-myr cycles in sedimentary series. Changes in phasing between carbonate production and insolation can suppress the low frequencies and favor the dominance of the 100-kyr eccentricity cycle. Consequently, in Tethyan Early Cretaceous deposits, clay-dominated marl-limestone alternations are associated to higher sedimentation rates and higher amplitudes of the long eccentricity cycles. Conversely, carbonate-dominated marl-limestone alternations are associated to lower sedimentation rates and higher amplitudes of the 100-kyr eccentricity cycles.