Geologic controls on phytoplankton elemental composition

Abstract

Planktonic organic matter forms the base of the marine food web, and its nutrient content (C:N:P_org) governs material and energy fluxes in the ocean. Over Earth history, C:N:P_org had a crucial role in marine metazoan evolution and global biogeochemical dynamics, but the geologic history of C:N:P_org is unknown, and it is often regarded constant at the "Redfield" ratio of ∼106:16:1. We calculated C:N:P_org through Phanerozoic time by including nutrient- and temperature-dependent C:N:P_org parameterizations in a model of the long-timescale biogeochemical cycles. We infer a decrease from high Paleozoic C:P_org and N:P_org 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.

Keywords: C:N:P; marine fauna evolution; organic matter stoichiometry.

Fig. 1.

Global temperature and surface–ocean phosphate concentration throughout the Phanerozoic (adapted from ref. 29). (A and B) The evolution of (A) temperature (K) and (B) surface–ocean phosphate concentration (µM P) obtained in ∼10⁶ default model simulations (5th to 95th percentiles of the results) where input parameters and time-dependent forcings were drawn from probability distributions that represent uncertainty in their values. Adapted from 29.

Fig. 2.

The evolution of phytoplankton C:N:P_org. (A and B) C:P_org (A) and N:P_org (B) (molar ratio) calculated from ∼10⁶ model simulations (5th to 95th percentiles of the results) where we draw ∼37 input parameters and 11 time-dependent forcings from probability distributions that represent uncertainty in their values. The elemental ratios were calculated using three parameterizations for the dependence of C:N:P_org on environmental parameters (Materials and Methods). Boxplots show the C:P_org and N:P_org of modern major phytoplankton groups (boxes, 25th, 50th, and 75th percentiles; whiskers, 99.3% of the data) growing under nutrient-replete conditions, plotted against their time of peak occurrence in the fossil record. (C) Occurrences of the different phytoplankton groups in the geologic record (Materials and Methods).

Fig. 3.

Effects of the proposed Phanerozoic evolution of C:N:P_org on the carbon and O₂ cycles. (A–H) Comparison between (A) C:P_org (molar ratio), (B) carbon-based primary productivity ($\times {10}^3$ Tmol C × y^–1), (C) marine organic carbon burial (Tmol C × y^–1), (D) weathering of organic matter in sedimentary rocks (Tmol C × y^–1), (E) atmospheric pO₂ (atm), (F) the deep-ocean O₂ concentration (µM), (G) atmospheric pCO₂ ($\times {10}^3$ ppm), and (H) temperature (K), for C:N:P_org that is fixed at the Redfield ratio (dashed black line) or variable according to the three parameterizations presented in Fig. 2.

Fig. 4.

Dynamic response of model variables to an increase in the phosphate delivery rate at a constant (Redfield) and variable C:N:P_org. (A–F) Surface phosphate concentration (forcing function) (A), C:P_org of the primary producers (mol C ×mol P^–1) (B), carbon-based primary productivity (C), burial of organic carbon (D), atmospheric pO₂ (E), and atmospheric pCO₂ (F). All fluxes and pools are normalized to present-day values and are unitless.