Middle Eocene—Lower Oligocene Climatic Transition and Planktonic Foraminiferal Biostratigraphy at DSDP Sites 219 and 237, Arabian Sea and Western Tropical Indian Ocean

Abstract

The study of late Middle Eocene-Early Oligocene planktonic foraminifera at the Arabian Sea DSDP Site 219 and the Site 237, western tropical Indian Ocean has led to significant advancement over the sporadic and sketchy Eocene-Oligocene biostratigraphy described earlier. Orbulinoides beckmanni Zone (E12) that serves as the biostratigraphic proxy for climatic warming phase, the middle Eocene climatic optimum (MECO), is recorded at the Site 237. Sudden warming resulted in the extinction of Acarinina bullbrooki and A. topilensis at the begining and decline of large acarininids at the top of the Zone E12 with the extinction of muricate surface dweller species A. praetopilensis and Morozovelloides coronatus and M. lehneri. Contrary to the conventional attribution to cooling for which there is no geochemical evidence, eutrophication of species in the warm interval approximating the biozone E12 best explains the decline and extinction of muricate large acarininids and morozovelloidids close to the MECO warming event. Although the onset of MECO is marked by the appearance of Orbulinoides beckmanni its culmination is coincident with the extinction of, Globigerinatheka kugleri, G. euganea, Morozovelloides coronatus, M. lehneri and O. beckmanni. Thus, there is a distinctive episode of extinction of five species close to the end of MECO with the drop in temperature. The change from the greenhouse to the icehouse world occurred during the latest Eocene–Oligocene Transition (EOT) ~34 Ma is expressed by the composition and pattern of change in the planktonic foraminiferal assemblages of the zones E13 to O2 at the two Sites. Almost complete Eocene-Oligocene stratigraphic record from zones E12 to O2 at the Deep Sea Sites studied in tropical Indian Ocean is consistent with other global reference sections. The late Eocene cooling and continental glaciation on Antarctica effected successive closely spaced events of extinction of cerroazulensis group of species and hantkeninids; only the first pulse of extinction of cerroazulensis group of species is recorded at the two Sites as the hantkeninid extinction at the E/O boundary is not documented in the present work due to the nonrecovery or non-availability of core samples. The significant bioevent documented here at the two Sites is the early origin of Globoquadrina tapuriensis, G. selli and G. venezuelana in the late Eocene. Similar early appearance of these globoquadrines recorded from Tanzania, Spain and Italy extend down their range to Late Eocene.