Phosphorus is crucial for reconstructing long-term feedback mechanisms between climate, the environment and ecology, as well as for assessing global biogeochemical changes. This study documents two thin yet laterally continuous phosphorite beds from the lower Nenjiang Formation (Late Cretaceous) of the Songliao Basin in NE China and evaluates their mineralogical characteristics and paleoenvironmental significance. The phosphorite beds occur in sharp contact with adjacent black shale and contain well-preserved Ostracoda and conchostracan fossils, providing biological constraints on the depositional conditions. Bulk rock compositions indicate elevated P2O5 contents, ranging from approximately 20 to 30 wt%. Mineralogical analyses reveal that the dominant phosphate mineral is carbonate-fluorapatite (CFA), accompanied by minor quartz, hydromica, goethite and pyrrhotite. Integrated fossil, sedimentological, and geochemical evidence suggests that CFA precipitated in a deep, stratified, eutrophic lacustrine environment. Enhanced productivity, biological enrichment and microbial decomposition of organic matter likely promoted phosphorus enrichment in bottom waters, facilitating CFA precipitation at or near the sediment-water interface during deposition and early diagenesis. Variations in physicochemical conditions, including pH and Ca2+ concentrations, may have further influenced mineral precipitation and subsequent diagenetic processes. These findings contribute to our understanding of phosphorus precipitation mechanisms in lacustrine basins and provide new constraints on the Late Cretaceous paleoenvironment of the Songliao Basin.
Liu et al. (Tue,) studied this question.