EFFECTS OF GEOLOGICALLY SHORT-LIVED SURFACE WATER DISRUPTIONS ON THE EVOLUTION OF CALCAREOUS NANNOPLANKTON AS EXEMPLIFIED BY OCEANIC ANOXIC EVENT OAE-1D AND THE PALEOCENE-EOCENE THERMAL MAXIMUM

Calcareous nannoplankton are obligate photoautotrophs restricted to the photic zone in the open ocean. Disruptions to the chemical and physical structure of the surface water mass constitute the most important environmental stresses forcing evolutionary changes. Relatively minor perturbations in the structure of the upper surface water mass, such as that associated with mid-Cretaceous oceanic anoxic event 1d, led to selective extinction of morphologically specialized oligotrophic taxa that were dependent upon maintaining a fixed position in a stable, stratified water column. These taxa were relatively rare components of the late Albian assemblages. In addition, the environmental changes associated with the anoxic event were coincident with originations and extinctions of less specialized clades undergoing adaptive radiation. A more extensive perturbation of the surface water mass, exemplified by the Paleocene-Eocene Thermal Maximum (PETM) resulted in higher rates of taxonomic turnover that affected both oligotrophic and mesotrophic species. These taxa were relatively minor parts of the total assemblages, however, suggesting that the numerically abundant species were not affected significantly by the event. These examples suggest that geologically brief disruptions of the upper water column preferentially affect rare taxa, while the more common calcareous nannoplankton exhibit a remarkable resilience to the effects of all but the most extreme short-lived perturbations of the surface water mass.