Geologic controls on phytoplankton elemental composition

Planktonic organic matter forms the base of the marine food web, and its nutrient content (C:N:Porg) governs material and energy fluxes in the ocean. Over Earth history, C:N:Porg had a crucial role in marine metazoan evolution and global biogeochemical dynamics, but the geologic history of C:N:Porg is unknown, and it is often regarded constant at the “Redfield” ratio of ∼106:16:1. We calculated C:N:Porg through Phanerozoic time by including nutrient- and temperature-dependent C:N:Porg parameterizations in a model of the long-timescale biogeochemical cycles. We infer a decrease from high Paleozoic C:Porg and N:Porg to present-day ratios, which stems from a decrease in the global average temperature and an increase in seawater phosphate availability. These changes in the phytoplankton’s growth environment were driven by various Phanerozoic events: specifically, the middle to late Paleozoic expansion of land plants and the Triassic breakup of the supercontinent Pangaea, which increased continental weatherability and the fluxes of weathering-derived phosphate to the oceans. The resulting increase in the nutrient content of planktonic organic matter likely impacted the evolution of marine fauna and global biogeochemistry.