DID MANTLE-DERIVED PHOSPHORUS FERTILIZE THE CRYOGENIAN OCEAN?

Elevated marine phosphate levels linked to an increase in biologic activity in the Cryogenian can be explained by the erosion of large subaerial igneous provinces. Preserved basalts have high enough phosphorus concentrations that a single 1.5M km³ igneous province (comparable in size to the Karoo-Ferrar) could have contained 75 times the dissolved P in the modern ocean. The phosphorus-rich basalts may be a consequence of a global transition to cooler crustal subduction in the Neoproterozoic. Specifically, the decarbonation of metasediments in subduction zones likely transitioned from the shallow forearc to the deeper backarc, causing widespread CO₂ metasomatism of subcontinental mantle. Phosphorus strongly partitions from silicate minerals into carbon-rich fluids in the mantle and precipitates from metasomatic fluids as apatite, forming phosphorus-rich zones. If phosphorus was broadly redistributed into such zones beneath the supercontinent Rodinia, large igneous province magmas probably incorporated metasomatic apatite and transported mantle phosphorus to the surface; marine sediments preserve low ⁸⁷Sr/⁸⁶Sr and elevated ¹⁴³Nd/¹⁴⁴Nd, consistent with large volumes of mantle-derived rock eroding into the ocean prior to the Sturtian glaciation. The breakup of Rodinia was punctuated by multiple large igneous events, but early rift magmas would be expected to have the largest metasomatized mantle component—and, thus, the highest phosphorus content—and would have been most susceptible to subaerial weathering. This potential link between tectonics and seawater composition suggests that the bioavailability of phosphorus on Earth over time may have been influenced by the progressive cooling of crustal subduction, as well as the supercontinent cycle.