Phosphorus And Iron Speciation In Ningxiang-type Iron Ore

As an essential element for all life,phosphorus（P）controlled marine primary productivity on geological timescales.The disturbance of biogeochemical cycles of P during the Late Devonian time was considered to have played important roles in Frasnian-Famennian（F-F）marine anoxia and sedimentary formations,including black shales and oolitic ironstones（OIS）.Speciation of P in sediments is sensitive to environmental conditions.Therefore,a detailed study on P speciation in ancient sediments,particularly the relative stability of the P pools and their transformation,may help reconstruct biogeochemical cycles of P and decipher the roles of P in large geological events.However,it was technically challenging to differentiate P species in ancient sedimentary rocks,especially those with high iron（Fe）content.Herein,a combination of improved sequential P extraction procedure（P-SEDEX）and spectroscopic techniques,including ³¹P solid-state nuclear magnetic resonance(³¹P SS-NMR),P X-ray absorption near edge structure spectroscopy（P-XANES）,and Fe extended X-ray absorption fine structure（P-EXAFS）,were used to examine P speciation of OIS collected from Huoshaoping Town,Changyang County,Yichang,Hubei,China.Our results suggested that the sedimentary P was composed predominantly of Fe-P,Al-P and Ca-P.Specifically,samples from different OIS layers formed during the process of transgression were distinguished by varying proportions of the three P speciation.More importantly,a combination of remarkable shifts from Al-P&Fe-P to Ca-P and high TOC/P_org（>106:1）,as well as extremely low TOC/P_reac（<<106:1）across the Frasnian-Famennian boundary suggest a strong disturbance of biogeochemical cycles of P happened during this period.The change from oxic bottom water condition to anoxic bottom water condition caused by transgression was supposed to be the main driving force for the disturbance of biogeochemical cycles of P.In this study,the molecular speciation of P and Fe was analyzed to further reveal the perturbation of biogeochemical cycle of P during the Late Devonian,and provide a possible explanation for the relationship between the oceanic anoxia event and sedimentary formations,including black shales and OIS.