The C–S–Fe system evolution reveals organic matter preservation in lacustrine shales of Yanchang Formation, Ordos Basin, China

Dissimilatory iron reduction (DIR) and bacterial sulfate reduction (BSR) are vital links in the cycle of the C–S–Fe system. They have been extensively studied in relation to the preservation of organic matter (OM) in marine anoxic environments. However, there has been little attention on DIR and BSR in ancient continental lakes with complex geologic factors. This paper tentatively reveals the differential OM enrichment of lacustrine shale in the Triassic Chang 7 Member of Ordos Basin. The dynamic evolution of the C–S–Fe system is investigated by mean of element geochemistry, a sequential extraction procedure for iron components, and cluster analysis. The results show that (1) the BSR triggered by the exogenous-inputting sulfate formed H2S in continental lake basins, and the enhanced H2S concentration which prompted the strongly anoxic body is particularly conducive for the preservation of OM in the Chang 7 Member. (2) Deposition of the Triassic Chang 7 Member organic-rich shale took place in a predominantly anoxic ferruginous environment. The reactive irons of pyrites (Fepy) and siderites (Fecarb) were produced during the mineralization of organic matter. The components of the Fepy and Fecarb reflect orderly dynamic evolution of the interaction between BSR and DIR. With the increase of sulfate input, the BSR reaction intensity increased, while DIR reaction intensity decreased. That can be quantitatively characterized by the value of Fepy/FeHR.