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.δ ¹³C measurements on the carbonate fraction show a 3%₀ depletion from the Cambrian to the Mississippian and a 1–2%₀ enrichment to the Jurassic. Both the sign of these displacements and a good correlation with the organic carbon content suggest that real variations in the rate of photosynthesis may have been the driving force for these changes with age.Because of the large isotopic heterogeneities in the organic biomass, systematic fluctuations with age in the δ ¹³C-values of the total organic carbon record are hard to prove. Nevertheless, a significant 3%₀ 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.Comparison of coexisting δ ¹³C_org?δ ¹³C_bit pairs demonstrates that during the Ordovician-Mississippian time period no significant (> 1%₀) differences in the isotope values of these two organic carbon sources 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, i.e. the depth of burial of these sediments, the $\text{1}\text{2}$(pristane/n-C₁₇ + phytane/n-C₁₈) ratio and the n-alkane distribution pattern support the theory that the rank of the organic matter may be responsible for the observed isotopic pattern.