Distinct Responses of Marine Phytoplanktons to the Asteroid Impact and Volcanism at the Cretaceous-Paleogene Boundary (K-Pg)

One of the prominent features of the Cretaceous/Paleocene (K-Pg) mass extinction is the highly selective extinction of marine phytoplankton. It is generally thought that the disturbances in the marine biogeochemical processes were caused by the asteroid impact in the Yucatan and/or the Deccan volcanism. However, which one is dominant remains debated. Here, we use Community Earth System Model (CESM1.2.2) to explore their influences on the latest Cretaceous marine biosphere. It is found that the asteroid impact led to a decrease by ~32% in the calcareous algae but an increase of ~95% in the diatom, consistent with the divergent trends of the abundance of calcium carbonate and biogenic silica archived in marine sediments. The rapid decline of the calcareous algae was because of lowered temperature and decreased light due to the asteroid impact, whereas the increase of diatoms was induced by the input of impact-generated debris and enhanced vertical mixing of the surface ocean, both of which increased nutrient supply. The counteraction between calcareous algae and diatom, to some extent, ensured the resilience of the bulk ocean biogeochemical cycle to the asteroid impact in the latest Cretaceous, with the total biomass increasing by ~2.7%. In comparison, the long-term forcing  (CO₂-induced warming) due to Deccan volcanism reduced 10-20% of all types of phytoplanktons. The trend and magnitude of this change are significantly different from that triggered by the asteroid impact, suggesting that asteroid impact was more likely the primary driver of the selective phytoplankton extinction at the K-Pg boundary.