Marine Phosphate Level During the Archean Constrained by the Global Redox Budget

Understanding the oceanic phosphate concentration is critical for understanding marine productivity and oxygen evolution throughout Earth history. During the Archean, estimates of marine phosphate levels range from scarce to enriched conditions. However, biogeochemical conditions required for sustaining high phosphate concentrations while retaining an anoxic atmosphere during the Archean remain ambiguous. Here, we employ a biogeochemical model of the marine phosphate cycle to determine the conditions under which oceanic phosphate levels could have been higher than present-day values during the Archean after the emergence of oxygenic photoautotrophs (OP). We show that, under the presence of OP, phosphate-rich oceans require the limitation by factors other than phosphate, or a high outgassing rate of reducing gases. If these conditions were not met, the occurrence of oceanic phosphate levels higher than present-day values during the Archean would require the absence of OP. The availability of phosphorus, a limiting nutrient in the ocean, is critical for understanding marine productivity in the history of the Earth, and hence atmospheric oxygen levels. On early Earth, the atmospheric oxygen level was much lower than the present conditions, and marine phosphate concentrations have been lower than the present level. However, an emerging proxy of the past phosphate level indicates a phosphate-enriched condition, which may contradict low atmospheric oxygen levels. To reveal the conditions required for sustaining high phosphate concentrations while retaining an anoxic atmosphere, we employed a theoretical model of the marine phosphate cycle. We show that, if oxygenic photosynthesis has evolved on early Earth, the primary productivity needs to be limited by factors other than phosphorus, such as nitrogen, or a high outgassing rate of reducing gases from volcanoes would be required for achieving high marine phosphate concentrations. If these conditions were not met, phosphate-rich conditions on early Earth would be limited to the periods before the evolution of oxygenic photosynthesis. The marine phosphate levels on early Earth after the evolution of oxygenic photosynthesis are estimated using a theoretical model Phosphate-rich oceans cause atmospheric oxygenation under reasonable outgassing rates of reducing gases Without suitable conditions, absence of oxygenic photosynthesis would be required for the phosphate-rich oceans during the Archean