The evolution of the marine phosphate reservoir

Phosphorus is generally thought to be a limiting nutrient of primary productivity in the oceans, and is important in regulating the redox state of the ocean–atmosphere system. Planavsky et al. use the ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks through time to evaluate the evolution of the marine phosphate reservoir. They find relatively constant phosphate concentrations during the past 542 million years of Earth's history. The data are also indicative of high dissolved phosphate concentrations in the aftermath of the 'snowball Earth' glaciations around 700 million years ago, which could have led to high rates of primary productivity, organic carbon burial and an increase in atmospheric oxygen levels, paving the way for the rise of metazoan life. Phosphorus is a biolimiting nutrient that is important in regulating the redox state of the ocean–atmosphere system. Here, the ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks through time has been used to evaluate the evolution of the marine phosphate reservoir. Phosphate concentrations have been relatively constant over the past 542 million years of Earth's history, but were high in the aftermath of the 'snowball Earth' glaciations some 750 to 635 million years ago, with implications for the rise of metazoan life. Phosphorus is a biolimiting nutrient that has an important role in regulating the burial of organic matter and the redox state of the ocean–atmosphere system1. The ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks can be used to track dissolved phosphate concentrations if the dissolved silica concentration of sea water is estimated2,3,4,5. Here we present iron and phosphorus concentration ratios from distal hydrothermal sediments and iron formations through time to study the evolution of the marine phosphate reservoir. The data suggest that phosphate concentrations have been relatively constant over the Phanerozoic eon, the past 542 million years (Myr) of Earth’s history. In contrast, phosphate concentrations seem to have been elevated in Precambrian oceans. Specifically, there is a peak in phosphorus-to-iron ratios in Neoproterozoic iron formations dating from ∼750 to ∼635 Myr ago, indicating unusually high dissolved phosphate concentrations in the aftermath of widespread, low-latitude ‘snowball Earth’ glaciations. An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere. The snowball Earth glaciations and Neoproterozoic oxidation are both suggested as triggers for the evolution and radiation of metazoans6,7. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.