Late Neogene planktic foraminiferal biochronology of the ODP site 763a, Exmouth Plateau, southeast Indian Ocean

摘要

The late Neogene section of ODP Hole 763A, Exmouth Plateau, southeast Indian Ocean, has been biostratigraphically subdivided into eight planktic foraminiferal zones. The zones are similar to those of the tropical northern Indian and tropical western Pacific Oceans. The sequential order of planktic foraminiferal events has been determined, and 32 late Neogene planktic foraminiferal events have been identified. A major faunal turnover (22 events) occurs between 80 and 45 meters below seafloor (mbsf), spanning 1.6 million years between 3.6 Ma and 2 Ma and may be related to the stresses transmitted to the upper ocean due to major cooling associated with Northern Hemisphere glaciation. Based on integration with magnetostratigraphy, numerical age estimates of planktic foraminiferal events have been determined for the first time from the southeast Indian Ocean. A comparison with the published dates for these events from other parts of the world suggests that 14 events are reliable for correlation over a wide geographic range. The useful synchronous events are the Globorotalia tosaensis FO (2.84 Ma), the Globorotalia tosaensis LO (0.59 Ma), the Globigerinoides extremus LO (1.87 Ma), the Globigerinoides fistulosits FO (3.26 Ma), the Globigerinoides fistulosus LO (1.73 Ma), the Dentoglobigerina altispira LO (3.05 Ma), the Dentoglobigerina globosa LO (3.18), the Neogloboquadrina dutertrei FO (2.77 Ma), the Sphaeroidinellopsis LO (3.35), the Globorotalia margaritae LO (3.38 Ma), the Globorotalia crassaformis FO (4.41 Ma), the Globoturborotalita nepenthes LO (3.44 Ma), the Globorotalia tumida tumida FO (5.10 Ma) and the Pulleniatina primalis FO (5.70 Ma). In contrast, the other events are diachronous and considered unsuitable for inter-oceanic correlation. The Pliocene-Pleistocene boundary is marked at ODP Hole 763A by the LO of Globigerinoides fistulosus, which is a synchronous event over a wide latitudinal range and occurs close to the top of the Olduvai Event (Chron C2n; 1.77 Ma). The Miocene-Pliocene boundary is approximated by the first evolutionary appearance of Globorotalia tumida tumida from its ancestor Globorotalia plesiotumida and occurs close to base of Thvera magnetic event (C3n.4n; 5.23 Ma). Recognition of both of these epoch boundaries is in accord with the International Commission on Stratigraphy. The detailed planktic foraminiferal biochronology developed for Hole 763A will aid correlation and dating of various paleoceanographic events over the last 6 million years in the southeast Indian Ocean.