Biomass recycling and Earth's early phosphorus cycle

Abstract

Phosphorus sets the pace of marine biological productivity on geological time scales. Recent estimates of Precambrian phosphorus levels suggest a severe deficit of this macronutrient, with the depletion attributed to scavenging by iron minerals. We propose that the size of the marine phosphorus reservoir was instead constrained by muted liberation of phosphorus during the remineralization of biomass. In the modern ocean, most biomass-bound phosphorus gets aerobically recycled; but a dearth of oxidizing power in Earth's early oceans would have limited the stoichiometric capacity for remineralization, particularly during the Archean. The resulting low phosphorus concentrations would have substantially hampered primary productivity, contributing to the delayed rise of atmospheric oxygen.

Fig. 1. Redox evolution of the Earth.

Compilations of (A) electron acceptor availability in seawater and (B) sedimentary organic carbon isotope record. The prevalence of extremely negative δ¹³C values from ~2.8 to 2.5 Ga has been interpreted by some as a signal of widespread methanogenesis [(90) and references therein].

Fig. 2. Total possible P recycling through geologic time.

Black line indicates preferred values. Gray shaded area is uncertainty envelope for C/P ratios of 106:1. Blue shaded region is uncertainty envelope for C/P ratios of 400:1; red shaded region corresponds to C/P of 1000:1. Dotted line shows modern concentration of P in the deep ocean and upwelling water (~2 μM).

Fig. 3. Total possible Archean P recycling as a function of ferric iron and sulfate availability.

Calculations are presented for C/P ratios of (A) 106, (B) 400, and (C) 1000. Diamond shows preferred values; pink shaded region shows published range of estimates for Archean seawater. Ferric iron reduction could have played a large role in P recycling if bioavailable Fe³⁺ levels were ~1 mM, but this scenario is very unlikely (discussed in the text). Elevated C/P ratios in primary producers would have severely impeded P recycling in all scenarios.

Fig. 4. Total possible Proterozoic P recycling as a function of sulfate availability.

Blue shaded region shows range of published estimates for Proterozoic sulfate concentrations. An increase in seawater sulfate levels after the GOE would have considerably increased the capacity for P recycling, although high C/P ratios could still have kept P levels low at the lower end of published estimates.