Iron and Phosphorus Cycling under Ferruginous Conditions

Considering the entirety of Earth’s history, the ocean has dominantly been in an anoxic, non-sulphidic state, with the water column containing dissolved iron (termed ‘ferruginous’). Despite the significance of such conditions, the cycling of nutrients in ferruginous settings remains poorly understood. Phosphorus (P) is thought to be the ultimate limiting nutrient on geological timescales. Therefore it is essential to constrain the behaviour of P under ferruginous conditions in order to evaluate feedback mechanisms associated with P stimulation of primary productivity, organic carbon burial and ultimately oxygen production. This study aims to further our understanding of iron (Fe) and P cycling under ferruginous conditions, through the investigation of a modern ferruginous lake and ancient sedimentary rocks deposited under ferruginous conditions. Modifications are made to the SEDEX P sequential extraction method to make it suitable for use with ancient sedimentary rocks and Fe-rich sediments. Using this newly revised method, the previously unidentified magnetite-bound pool of P is shown to have constituted a significant proportion of reactive P in many ancient ferruginous environments, indicating that magnetite may require further consideration when modelling P cycling under ferruginous conditions. Detailed investigation of the water column and sediments of the ferruginous Lake La Cruz reveals that whilst a substantial amount of P is released from the sediments, the formation of reduced Fe phosphates in the water column exerts a strong control on the vertical extent of the benthic P flux, a process that is most likely enhanced over the deeper water column depths of ancient ferruginous oceans. P partitioning in ancient sedimentary rocks from the ~1.8 billion year old Animikie Basin reveals that under stable redox environments, ferruginous conditions cause significant drawdown and burial of P, most likely resulting in a P limited water column. Whilst the majority of this P is now present as authigenic calcium phosphates, the presence of magnetite-bound P and observations of the modern ferruginous Lake La Cruz suggest this was most likely initially drawn down in association with Fe minerals.