Organic matter accumulations in the Santonian-Campanian (Upper Cretaceous) lacustrine Nenjiang shale (K2n) in the Songliao Basin, NE China: Terrestrial responses to OAE3?

Abstract

The local and/or global environmental effects on the organic matter (OM) enrichment in ancient sediments has long received extensive attention. The Upper Cretaceous Nenjiang lacustrine shale (K2n) is one of the most essential hydrocarbon source rocks in NE China with massive OM deposition across the Santonian–Campanian boundary interval, which can be correlated with the terminal stage of the last oceanic anoxic event in the Cretaceous, OAE3. This study focuses on the lower part of the Nenjiang Formation from drill core A-1 in the Songliao Basin paleo-lake system, and highlights two episodes of enhanced OM accumulation controlled by distinct mechanisms. During the first episode in Member 1 of K2n (K2n1), total organic carbon (TOC) values reach a maximum of ~8.0% but fluctuate rapidly. The second episode occurs at the base of the Member 2 of K2n (K2n2) and is associated with a more transient and rapid OM enrichment event exhibited as a single-peak TOC pulse (maximum ~13%). Inorganic and organic geochemical indicators consistently show that two OM accumulation episodes are controlled by different environmental backgrounds: (1) the pristane/phytane values, homohopane index, and the distribution of the redox-sensitive elements suggest that the oxygen content of the bottom water during the K2n1 was relatively lower with transient euxinia (anoxic and sulfidic water column); (2) the paleoproductivity and OM source indicators illustrated that marine-sourced OM occurred during K2n1, whereas terrestrial paleoproductivity had a greater impact on the OM accumulation during K2n2; (3) the water connectivity indicators demonstrate strong periodic seawater incursions during the K2n1, yet the environment during the early stage of K2n2 is more restricted and the paleolake was dominated by fresh-water. It is also underscored that terrestrial envi ronments might be more sensitive to climatic perturbations associated with OAEs. The rapid sea-level (lake-level) variations possibly controlled the distinct OM types and accumulation mechanisms and resulted in different OM accumulation patterns in the lower part of the K2n during the Santonian–Campanian. The widely distributed OM-rich sediments of the K2n in the Songliao Basin indicate that the enhanced burial of OM in terrestrial environment, along with similar coeval OM-rich deposits worldwide, can create important negative feedbacks to changes in the global exogenic carbon cycle possibly contributing to OAE3.