Effect of dissolved oxygen concentration on planktonic foraminifera through laboratory culture experiments and implications for oceanic anoxic events

Although substantial turnovers of planktonic foraminiferal species occurred during Cretaceous oceanic anoxic events (OAEs), the direct effects of dissolved oxygen (DO) concentration on planktonic foraminifera remain obscure. Culture experiments can quantify the relationship between planktonic foraminiferal ecology and environmental parameters, but experiments controlling DO have yet to be conducted because it is difficult to maintain a stable oxygen concentration. In this study, we cultured two planktonic foraminifer species (Orbulina universa and Globigerina bulloides) at six different DO levels between 10% and 100% saturation. Both species showed a high tolerance to low DO, suggesting that "dysoxic" conditions (>0.7mgO₂l^-1) were not a direct cause of planktonic foraminiferal extinction at OAEs. The high tolerance of these species to extremely low DO might be attributable to their evolutionary descent from benthic foraminifera. Final shell weight increased with increasing DO, suggesting that fossil foraminiferal shell weight could vary with past DO conditions. Our results suggest that the extinction of many planktonic foraminiferal species during OAEs may have been due to anoxic or euxinic conditions in the euphotic zone. The occurrence of these conditions can be explained either by the oxygen minimum layer model or by the stagnant ocean model combined with elevated riverine P input.