Evaluating the roles of alkalinity, evaporation, and basin hydrology in phosphorite deposition

Phosphorus, an essential element for all forms of life, is recognized as the ultimate limiting nutrient controlling marine primary production over geological timescales. Phosphorites, which are the most phosphorus-rich sedimentary deposits, therefore serve as a valuable archive for reconstructing past marine phosphorus cycling. However, the origin of ancient phosphorites—whether their occurrences reflect changes in the global seawater nutrient inventory or are instead tied to local depositional conditions—remains debated. To shed light on this question, we examined the Permian Phosphoria Formation, a phosphorus-rich sequence deposited along western Pangaea. While previous models attribute extensive phosphogenesis to open ocean upwelling along a continental margin as in modern analogs, trace element concentration data suggest that Phosphoria environments were instead (semi)-restricted. Notably, bulk sediments exhibit exceptionally high δ¹⁵N values (> +15‰). This may result from NH3 volatilization, implying unique local water mass chemistry with elevated pH and alkalinity. Alkaline conditions, further enhanced by vigorous evaporation, could have promoted phosphate accumulation in the water column. We propose an alternating redox model to explain the observed pattern of phosphate enrichment in the sediments. Our study highlights the importance of local factors—specifically alkalinity, evaporation, and basin restriction—in driving phosphogenesis in the Phosphoria Formation, with implications for other phosphorus-rich successions in deep time. These insights may help illuminate the temporal clustering of phosphorites in the geologic record, specifically during the early and late stages of the Proterozoic.