INSIGHTS INTO PHOSPHOGENESIS FROM A MULTI-PROXY PALEO-REDOX RECONSTRUCTION OF THE PERMIAN PHOSPHORIA BASIN, IDAHO, USA

Phosphorus is the ultimate limiting nutrient, controlling primary productivity and organic carbon deposition on geologic timescales. The cycling and deposition of phosphorus is controlled by a delicate balance of diagenetic processes that reflect redox and depositional conditions. The role of marine redox conditions is not well understood especially as it relates to the accumulation of extensive phosphorite deposits in shallow epeiric seas. Phosphorus-rich rocks are not common in the geologic record or modern sediments thus there is limited opportunity to investigate the related depositional processes.

Here, we utilize the phosphorite deposits of the Permian Phosphoria Rock Complex (PRC) as a case study. Previous reconstructions of the Phosphoria Basin, as well as published trace and rare earth element (REE) concentration records, suggest that upwelling of nutrient-rich waters was both the source of phosphorus and the driver of the dominantly suboxic conditions that led to phosphogenesis. From four sections in southeast Idaho, we analyzed bulk-rock iron speciation, organic carbon and trace element concentrations, as well as apatite-specific cerium anomalies and REE concentrations to better constrain local redox conditions and their role in the deposition of phosphorite. The high ratio of reactive iron relative to total iron during intervals of elevated phosphorite deposition suggests the shuttling of iron within the Phosphoria Basin. This supports the idea that the iron reduction zone must have been at or near the sediment-water interface during phosphorite accumulation. In modern systems, sulfur oxidizing bacteria can serve to help concentrate phosphorus as well, leading to enhanced burial of apatite. The extremely low pyrite concentrations in the PRC sections could be indicative of microbial sulfide oxidation in sediment porewaters. Apatite-specific cerium anomaly and REE data suggest a well-oxygenated water column during the deposition of the PRC. Together with the bulk-rock redox proxy data, this suggests a dynamic water column redox state. This combination of proxies allows for a unique opportunity to refine redox conditions in both the sediment porewaters, which would have been the primary influence on phosphogenesis, and the water column.