WAS THE MODERN PHOSPHORUS BIOGEOCHEMICAL CYCLE ESTABLISHED BY THE PALEOPROTEROZOIC?

The overwhelming majority of phosphorus in the oceans today is derived from continental weathering and delivered via rivers to shallow shelf areas. Areas with high phosphorus fluxes are typically characterized by high marine productivity and areas with low fluxes are often nutrient limited. When and how was this pattern established? Three competing hypotheses include: 1) the pattern was established early, with the first emergence of significant continental land masses in the Archean, 2) the pattern was established late, with the rise of both vascular and non-vascular land plants in the Ordovician, or 3) the pattern was established in the Proterozoic, perhaps coincident with either biological evolution or plate tectonic changes such as the assembly of Rodinia. To evaluate each of these competing hypotheses, a record of phosphorus weathering has been established using bedrock-parented paleosols. While phosphorus may be leached or retained for the Archean and Paleoproterozoic, after ca. 1.8 Ga paleosols are similar to modern soils in that they retain phosphorus quantitatively and even may show (bio-)accumulation at the tops of profiles. There is no change in the phosphorus content of the continents being weathered, so it is most likely that the change was triggered by a change in weathering intensity. The timing of this shift precludes either an Archean or Ordovician establishment of the modern phosphorus biogeochemical cycle and coincides with the last occurrence of Superior-style BIFs. Previously, workers have attempted to link this shift with nutrient limitation, which is supported by the new compilation. Similarly, while the new compilation does not preclude a Neoproterozoic “greening of the land”, it suggests an earlier fundamental change to the phosphorus biogeochemical cycle occurred first.