Cretaceous rhythmic bedding sequences: a plausible link between orbital variations and climate

Abstract

Simple oscillation cycles have been recognized in Cretaceous pelagic sequences as a marked periodicity in carbonate and/or organic carbon content. The estimated periodicities of these cycles resemble Milankovitch-type variations in orbital elements. However, during warm geologic periods the nature of the link between orbital variations and the sedimentary record has been problematic. Climate model studies show that the intensity of continental margin precipitation in specific low latitude regions is a function of land-sea thermal contrast which is sensitive to orbital variations. Cretaceous climate model simulations are characterized by regions of intense precipitation, the locations of which are controlled by geographic configuration. In particular, in the model simulation, the northern margin of the Tethys Ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations. Detailed studies of the biota, sedimentology and geochemistry of the bedding rhythms in Greenhorn Cyclothem in the Western Interior Seaway of North America are best interpreted in terms of variations in precipitation, runoff and stable stratification of the seaway. Combined the geologic and model studies support a link between orbital periodicities, variations of intense precipitation associated with specific paleogeographic situations and Cretaceous bedding patterns. This hypothesis has important implications for the sensitivity of the atmosphere-hydrosphere-lithosphere system to external forcing.