Stable isotope and organo-geochemical studies on Phanerozoic sediments of the Williston Basin, North America

Carbon isotope studies have been carried out on marine Phanerozoic sediments of the Williston Basin, North America. Special attention was given to the recognition of systematic variations with age. 13C abundance measurements show a 3 per mil. depletion from the Cambrian to the Mississippian and a 1-2 per mil. enrichment to the Jurassic. It is suggested that real variations in the rate of photosynthesis may have been the driving force for these changes with age. A significant 3 per mil. depletion was observed from the Devonian to the end of the Carboniferous. This depletion seems to occur on a worldwide scale. It is therefore proposed that a rise in the rate of photosynthesis as a consequence of the colonization of the continents by higher plants is the global cause of this displacement in the isotope record of the total organic carbon. The bitumen fraction does not show any systematic isotope variations with age. During the Ordovician-Mississippian time period no significant (>1 per mil.) differences in the isotope values of bitumen and organic matter can be observed. In contrast the bitumen fraction is depleted in 13C as compared to the total organic carbon in the Cambrian as well as in the Pennsylvanian-Cretaceous time interval. Several parameters support the theory that the rank of the organic matter may be responsible for the observed isotopic pattern. (Auth.)