Phosphorus (P) is a significant nutrient element which, by contributing to the modulation of primary productivity, has influenced oxygenic photosynthesis and thus global ocean redox conditions. Precambrian, especially mid-Proterozoic (1.8–0.8 Ga), seawater phosphorous concentrations have been poorly constrained. The well-preserved ~1.65 Ga ironstones in North China provide a nice opportunity to explore the phosphorous concentrations of shallow seawater at that time. Petrography and chemical analyses of the ironstones show that detrital apatite grains are abundant in the matrix of iron oolite and stromatolite. In addition, phosphorus concentrations in the laminasets of ooidal cortexes close to cracks and porous laminasets of stromatolitic columns are relatively high, suggesting that the diagenetic dissolution of detrital apatite grains and redistribution of phosphorus-rich fluids could increase phosphorus concentration in iron oxides. To avoid potential contamination we performed fabric-specific analysis of dense laminasets in ooid cortices without cracks, and in stromatolitic columns. These data revealed a low P/Fe molar ratio of 0.0007 ± 0.0003 (n = 12), pointing to an extremely low phosphorus concentration (<0.002–0.016 μM) in 1.65 Ga shallow seawater. Such a low phosphorus concentration would have limited primary productivity and therefore atmospheric oxygen levels during the terminal Paleoproterozoic.
Phosphorus (P) is a significant nutrient element which, by contributing to the modulation of primary productivity, has influenced oxygenic photosynthesis and thus global ocean redox conditions. Precambrian, especially mid-Proterozoic (1.8–0.8 Ga), seawater phosphorous concentrations have been poorly constrained. The well-preserved ~1.65 Ga ironstones in North China provide a nice opportunity to explore the phosphorous concentrations of shallow seawater at that time. Petrography and chemical analyses of the ironstones show that detrital apatite grains are abundant in the matrix of iron oolite and stromatolite. In addition, phosphorus concentrations in the laminasets of ooidal cortexes close to cracks and porous laminasets of stromatolitic columns are relatively high, suggesting that the diagenetic dissolution of detrital apatite grains and redistribution of phosphorus-rich fluids could increase phosphorus concentration in iron oxides. To avoid potential contamination we performed fabric-specific analysis of dense laminasets in ooid cortices without cracks, and in stromatolitic columns. These data revealed a low P/Fe molar ratio of 0.0007 ± 0.0003 (n = 12), pointing to an extremely low phosphorus concentration (<0.002–0.016 μM) in 1.65 Ga shallow seawater. Such a low phosphorus concentration would have limited primary productivity and therefore atmospheric oxygen levels during the terminal Paleoproterozoic.

ooidal ironstone,stromatolitic ironstone,P/Fe ratio,phosphorus concentration,primary productivity