The modern phosphorus cycle informs interpretations of Mesoproterozoic Era phosphorus dynamics

The cycles of phosphorus, carbon and oxygen are intimately linked. Indeed, in many models, phosphorus is considered the driver of the carbon and oxygen cycles, and low concentrations of atmospheric oxygen during the Mesoproterozoic Era have been linked to extreme phosphorus limitation in the Mesoproterozoic oceans. To evaluate the Mesoproterozoic Era phosphorus cycle, we analyze the concentrations of phosphorus, organic carbon, and selected trace metals in several geological formations of Mesoproterozoic age. We combine these analyses with literature data to explore the relationship between phosphorus and organic carbon removal into Mesoproterozoic Era sediments through a variety of water depths and water-column chemistries. We find that the ratio of organic carbon to reactive phosphorus (Corg/Preact) is largely invariant between different paleo-settings with average Corg/Preact that is either equal to or less than the Redfield ratio of 106/1 through all environments we explored. We put these results in the context of the modern phosphorus cycle which is reviewed here. Compared to modern phosphorus dynamics, we see no evidence for an anoxic-euxinic feedback between phosphorus burial, carbon burial and oxygen production during Mesoproterozoic times. However, we do identify an additional potential phosphorus feedback related to the relationship between anoxia and deep-sea phosphorus dynamics that could have importance in oxygen regulation through time. We find that the average value of Corg/Preact during the Mesoproterozoic Era was greater than the average for modern sediments. This result suggests that equal or more organic carbon was buried per unit of phosphorus during Mesoproterozoic times compared to today, a conclusion broadly consistent with the carbon isotope record. These results offer the possibility of a strong oxygen source to the atmosphere during the Mesoproterozoic Era, raising the conundrum as to why atmospheric oxygen levels were lower then when compared to now. We suggest that a variety of factors may explain these differences in oxygen concentration including elevated rates of mantle degassing, reduced rates of phosphorus weathering and the lack of a terrestrial biosphere.