Evolution of the global phosphorus cycle

Abstract

The macronutrient phosphorus is thought to limit primary productivity in the oceans on geological timescales. Although there has been a sustained effort to reconstruct the dynamics of the phosphorus cycle over the past 3.5 billion years, it remains uncertain whether phosphorus limitation persisted throughout Earth's history and therefore whether the phosphorus cycle has consistently modulated biospheric productivity and ocean-atmosphere oxygen levels over time. Here we present a compilation of phosphorus abundances in marine sedimentary rocks spanning the past 3.5 billion years. We find evidence for relatively low authigenic phosphorus burial in shallow marine environments until about 800 to 700 million years ago. Our interpretation of the database leads us to propose that limited marginal phosphorus burial before that time was linked to phosphorus biolimitation, resulting in elemental stoichiometries in primary producers that diverged strongly from the Redfield ratio (the atomic ratio of carbon, nitrogen and phosphorus found in phytoplankton). We place our phosphorus record in a quantitative biogeochemical model framework and find that a combination of enhanced phosphorus scavenging in anoxic, iron-rich oceans and a nutrient-based bistability in atmospheric oxygen levels could have resulted in a stable low-oxygen world. The combination of these factors may explain the protracted oxygenation of Earth's surface over the last 3.5 billion years of Earth history. However, our analysis also suggests that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic eon (between 800 and 635 million years ago), coincident with a previously inferred shift in marine redox states, severe perturbations to Earth's climate system, and the emergence of animals.

Early phosphorus redigested

“…Reinhard and colleagues 4 argue that low P concentrations in shallow marine shales deposited throughout the majority of early Earth history reflect enhanced scavenging from anoxic, ironrich oceans, leading to severe P biolimitation. This P scarcity before approximately 800 million years ago led to increased molar C/P ratios in the biomass of primary producers, with C/P ratios of 300:1 or more, compared to the modern Redfield ratio of 106:1.…”

“…These processes can result in significant fluxes of P back to the water column, substantially increasing C/P ratios in the deposited sediment 9 . Reinhard and colleagues 4 implicitly include a recycling component in their model, but the extent to which the degree of recycling has evolved through time is currently poorly understood. Sulphidic marginal marine environments became more prevalent through early Earth history 1,10 as a consequence of rising sulphate concentrations in the ocean, initially in association with possible 'whiffs' of oxygen prior to Earth's first persistent rise in atmospheric oxygen, and subsequently during Earth's 'middle ages' (Fig.…”

“…Reinhard and colleagues 4 interpret this as a bi-stability in atmospheric oxygen levels controlled by Plimitation, which resulted in a stable low-oxygen world prior to 800 million years ago. Unfortunately, however, model uncertainties make it difficult to constrain precisely how low atmospheric oxygen was in this low-oxygen world 4 .…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…Similarly, Reinhard et al (2017) presented a large compilation of P data from siliciclastic sediments and identified a four-fold increase in P burial at the start of the Cryogenian (~720 Ma), which they linked to progressive oxygenation of the Earth and loss of the deep-sea Fe trap. Poulton (2017) countered that the record presented by Reinhard et al (2017), and the large observed increase in P burial in the Neoproterozoic, may alternatively be explained by high P liberation and recycling from the sediment back into the water column. Accordingly, the extent to which P may have limited cyanobacterial productivity in the Proterozoic ocean remains unresolved.…”

“…Although P is likely to have been the ultimate limiting nutrient (e.g., Reinhard et al, 2017), limitation need not have been restricted to this element. With denitrification in the ocean interiors, N fixation in surface waters is needed to balance any P input to the oceans.…”