Geologic controls on phytoplankton elemental composition.

ABSTRACT

Planktonic organic matter forms the base of the marine food web, and its nutrient content (CNPorg) governs material and energy fluxes in the ocean. Over Earth history, CNPorg had a crucial role in marine metazoan evolution and global biogeochemical dynamics, but the geologic history of CNPorg is unknown, and it is often regarded constant at the "Redfield" ratio of ∼106161. We calculated CNPorg through Phanerozoic time by including nutrient- and temperature-dependent CNPorg parameterizations in a model of the long-timescale biogeochemical cycles. We infer a decrease from high Paleozoic CPorg and NPorg 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.