Effects of sources and diagenesis on the isotopic and chemical composition of carbon and sulfur in Cretaceous shales

Abstract

The Cretaceous Lea Park Formation of south-central Saskatchewan contains organic matter from marine and terrestrial sources in proportions that vary as a result of deposition during the Campanian Claggett transgressive-regressive cycle in the Western Interior seaway of North America. The relative proportions of organic matter from these sources were determined using a terrestrial index (TI) that compares n-alkanes considered to be of terrestrial origin, n- C 25 to n- C 59, to those considered to be of marine origin, n- C 15 to n- C 22. The C isotopic composition of Cretaceous marine organic matter is found to be near −32%o while terrestrial organic matter is near −24%.. The lower δ 13C values of marine organic matter from the Lea Park Formation, as compared with those of Recent marine organic matter, are similar to the relation observed in other Cretaceous rocks; δ 13C values of terrestrial organic matter, however, are similar to those of the present. During the Miocene, δ 13C values of marine organic matter became similar to Recent values, and this change in isotopic composition may be related to the development of deep cold-water associated with the formation of Antarctic ice sheets. Consequences of the formation of deep cold-water may include improved exchange of CO 2 between atmosphere and oceans, reduced pCO 2 levels and CO 2 concentrations in the photic zone of the oceans, and environmental changes sufficient to produce an alternate metabolic pathway in phytoplankton. All of these processes potentially result in higher δ 13C values in marine organic matter. Pyrite formation in the Lea Park mudstones was limited by the availability of metabolizable organic matter, with the greatest amount of pyrite typically associated with organic matter having the lowest TI values. δ 34Spy values range nearly 30%. and vary directly with the composition of associated organic matter. Variations in the supply of metabolizable organic matter were more important than differences in sedimentation rate in producing the range of δ 34Spy values. A 34S values (δ5 34Sseawater - δ 34Spyrite) vary with the composition of organic matter and become larger when associated with more metabolizable organic matter.