Response of oceanic organisms to abiotic events in the Paleogene

Climate fluctuations with the optimum in the Early Eocene and subsequent cooling were the main abiotic factor that controlled the development of the oceanic biota in the Paleogene. The Paleogene represented the transitional stage from the greenhouse climate of the Mesozoic to the partly glacial Neogene and was characterized by changes in the distribution of the temperatures in the ocean with the replacement of the dominant latitudinal thermal circulation by the largely meridional thermohaline one. The climate changes were also determined by other factors: the opening and closure of seaways between basins, the position of major currents, volcanic activity, the sea-level fluctuations, the composition of the hydro- and atmosphere, and others. These changes were, in turn, determined by factors of higher order, primarily, by tectonic movements: vertical and horizontal (motions of lithospheric plates). The contribution of impact events to this process is also highly probable. All these factors influenced, via the hydrological and hydrochemical parameters of the water column, the evolution of the oceanic biota: their distribution areas, the sizes of the organisms, the diversity of the communities, the bioproductivity, and the mass extinction (for example, the extinction of 30–50% of the benthic foraminifers at the Paleocene-Eocene transition in response to the abrupt temperature increase). The Eocene-Oligocene transition (38 Ma ago) was marked by a global biotic crisis, the most significant one in the Cenozoic, when the abyssal part of the ocean was filled with cold water to form the psychrosphere. At least five major impact events, which preceded the Oligocene mass extinction of the biota, occurred in the terminal Eocene (36–35 Ma ago).