Contrasting response of the calcareous nannoplankton communities after the Eocene hyperthermal events in the tropical Atlantic Ocean

Latest Paleocene and early Eocene hyperthermals were geologically brief, profound environmental perturbations caused by massive additions of 13C-depleted carbon to the ocean-atmosphere system, and therefore provide ancient analogs to assess the impact of modern global warming on the Earth's ecosystem. We document and compare population changes in calcareous nannoplankton, the major primary producers in Paleogene oceans, across three consecutive hyperthermal episodes of differing magnitudes (i.e., the most severe Paleocene/Eocene Thermal Maximum (PETM), the relatively mild Eocene Thermal Maximum 2 (ETM2) and the least severe H2) in the tropical Atlantic. Across all three hyperthermal events, the total abundance and the abundance of the eutrophic and mesotrophic taxa decreased significantly, while oligotrophic taxa increased markedly, suggesting that oligotrophy prevailed in tropical Atlantic surface waters. A nonmetric multidimensional scaling ordination revealed that, during the peak of each hyperthermal, nannoplankton communities featured excursion taxa and assemblages substantially deviated from the pre-event composition. However, during the recovery phase, nannoplankton communities rapidly returned to quasi pre-event compositions for the ETM2 and H2 perturbations, but remained distinct from the pre-event composition for the PETM. This contrast between pre- and post-event compositions of nannoplankton communities may imply that the PETM perturbation exceeded the resilience threshold of the tropical Atlantic surface ecosystem and caused a regime shift, cautioning the possible risk of an abrupt state change in oceanic ecosystem under the present situation, which has a considerably faster and greater perturbation to the C cycle than during the PETM.