A Greener but Less Productive Proterozoic Ocean

Geological records suggest that marine phytoplankton might have arisen in the Proterozoic while zooplankton remained absent, and marine productivity was not excessively low. However, quantitative estimates of phytoplankton biomass and net primary productivity (NPP) remain elusive. Here, we use the Earth system model CESM1.2.2, modifyingbiological module and boundary conditions, to simulate marine biogeochemical cycles in the Proterozoic. The simulations demonstrate that, within the expected range of nutrient levels, phytoplankton at sea surface was >2 times denser than present, sustaining a greener ocean due to the absence of predators. Heavier surface chlorophyll in the Proterozoic would block sunlight from penetrating subsurface layers. This so-called self-shielding effect would decrease subsurface NPP significantly. Simulations show that, through the combined influence of low nitrate level under a low-oxygen environment, the absence of diatoms, and self-shielding, the Proterozoic NPP was only ~60% and 30% of the present level in warm (almost ice-free) and cold (sea-ice reaches ~30°N/S) periods, respectively.