What drove the evolutionary trend of planktic foraminifers during the Cretaceous: Oceanic Anoxic Events (OAEs) directly affected it?

The oceanic redox state has played an essential role in the evolution of life on earth, and oceanic “anoxic events” have been proposed as one of the causal mechanisms for mass extinctions. During the mid-Cretaceous, oceanic anoxic events (OAEs) occurred several times and had a substantial impact on the biosphere. Planktic foraminifers are marine planktons with calcite tests, and their shells constitute approximately 30%–80% of modern deep-marine calcite; thus, they play a key role in the global carbon cycle. Previous studies have reported that at, or near, the times of major Cretaceous OAEs, planktic foraminifers have a high turnover (extinction and speciation) rate. However, the precise impact of anoxic conditions on the evolutionary trend of planktic foraminifers remained obscure. In this study, we investigate this by assessing the extinction/speciation rate of planktic foraminifers around OAEs. Our results suggest that the development of anoxia during OAEs would not have had a direct effect on the evolutionary trend (i.e., species number and turnover ratio). Two foraminiferal turnovers occurred around OAE1a and OAE2, however, all five turnover peaks seem to be largely associated with a relatively short-term (0.5–3 Myr) eustatic curve. Decreasing species numbers of planktic foraminifers are associated with a relatively lower sea eustatic level (second-order; ~20 Myr) and a small latitudinal temperature gradient around OAE1b, which are not anoxic conditions. Changes in oceanographical parameters (i.e., surface biological productivity, water column structure, carbonate chemistry, and nutrients) related to lowering sea-levels might have had a significant effect on foraminiferal evolutionary trends.