Iron–phosphorus–carbon coupled cycling in lacustrine system of Ordos Basin during the Middle Triassic

Widespread organic-rich shales are associated with perturbations in productivity or preservation conditions; however, the major controlling factor especially the phosphorus (P), iron (Fe) interactions and their impacts on organic carbon burial in the paleolake ecosystem remain poorly understood. Here, we report iron and phosphorus speciation data from the Chang 7 Member of Ordos Basin to elucidate iron–phosphorus–carbon cycling (Fe–P–C) in the freshwater lacustrine system during the Middle Triassic. Our results show that the redox conditions and phosphorus cycling exhibit strong spatiotemporal heterogeneity. During Interval I (0–140 kyr), oxic–ferruginous conditions were dominant in the water column, abundant Fe (oxyhydr)oxides promoted sedimentary P retention and sustained oligotrophic conditions. During Interval II (140–405 kyr), the widespread development of euxinic water bodies expanded to the slope belt of the lake basin, leading to extensive P recycling back to photic zone, thereby increasing primary productivity and organic matter production. This transition is probably attributed to enhanced volcanic activity and a significant shift in redox conditions, which play an important role in regulating the concentration of bioavailable P in the water column. This study sheds new light on Fe–P–C cycling in the ancient freshwater lake under volcanic perturbations, providing guidance on the link between biogeochemical feedbacks and carbon burial.