Dominant Dynamic Anoxic Ferruginous Conditions During the Paralic Lacustrine Organic Carbon Preservation of Middle Eocene in East China

The anoxic water column and the formation of organic carbon are strongly interlinked. Organic ligands have been shown to control the concentration of reactive iron mostly in the bottom of paleo-ocean. However, the role of iron in the preservation of organic carbon in sediments has not been clearly established in the paleo-lakes. As the depositional environment the lake is much smaller than the ocean, but much more sensitive. Also the environment of coastal lakes is influenced by the ocean transgression, the formation of organic carbon in organic matter is more complicated. Here we use the iron speciation, organic carbon isotope (δ 13 C org ) as well as pyrite sulphur isotope data (δ 34 S py ) to determine the relationship between the preservation of organic carbon and anoxic conditions in fine-grained sediments from the Middle Eocene lacustrine depositional environments of Jiyang Depression, Bohai Bay Basin, East China. Our findings suggest that the sediment of organic-enrich shales of Shahejie Formation in Jiyang Depression might correspond to the change of the global carbon cycle driven by the Middle Eocene Climatic Optimum (MECO), and the preservation of organic carbon in sediments is directly bound to ferruginous conditions. We further estimate that the ferruginous conditions in the relatively closed paleo-lake systems were initiated by an increase in the influx of highly reactive iron relative to sulphate, driven by a change in water column and the uptake of sulphate by extensive volcanism or transgression. We propose that ferruginous conditions were both spatially and extensive across diverse palaeogeographic settings in the paralic lacustrine system, promoted the long-term storage of organic carbon in sediments and thus contributed to the global cycles of carbon, sulphur and iron.