Coupling X-ray absorption spectroscopy and chemical extractions to investigate phosphorus phase partitioning in Ningxiang iron deposits

Phosphorus (P) is the ultimate limiting nutrient for primary productivity on geological timescales, but reconstructing ancient seawater phosphate concentrations in iron-rich sediments is complicated by post-depositional transformations that may obscure primary P signatures. Here, we investigate phosphorus phase partitioning in Late Devonian Ningxiang oolitic ironstones from South China, using a combined approach integrating sequential chemical extractions, solid-state 31P nuclear magnetic resonance (SSNMR), and P K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The ironstones, composed mainly of hematite, exhibit systematically higher P contents than their siliciclastic host rocks. Our results reveal pronounced stratigraphic variability in P phases: Al/Fe-bound P dominates in the Huangjiadeng Formation, whereas authigenic Ca-phosphates (apatite and carbonate-fluorapatite) prevail in the overlying Xiejingsi Formation. This transition reflects diagenetic transformation of primary Fe-P phases under progressively reducing pore-water conditions, followed by re-precipitation as stable Ca-phosphate minerals. Spectroscopic analyses reveal that Al-bearing minerals constitute a significant, yet previously underappreciated, sink for phosphorus. This redistribution of P among Fe-, Al- and Ca-bound phases challenges the conventional use of bulk P/Fe ratios in Phanerozoic ironstones as direct proxies for paleo-seawater phosphate concentrations. Our study demonstrates that coupling XAFS and NMR spectroscopy with sequential extractions provides a powerful approach to resolve phosphorus phase partitioning and cycling in ancient marine settings.