Tropical Radiolarian Biostratigraphy from the Early to Late Miocene at ODP Site 714 in the Tropical Indian Ocean

Abstract

Detailed biostratigraphic study of a lower Neogene section with abundant, moderate to well preserved radiolarians from central Indian Ocean Drilling Program (ODP) Site 714 has resulted in the precise correlation of many radiolarian bioevents with calcareous nannofossil and planktonic foraminiferal biostratigraphy. The interval studied at Site 714 is divided into five radiolarian zones, RN6 to RN2. In this study, Zones RN5 (Dorcadospyris alata Interval Zone), RN4 (Calocycletta costata Interval Zone) and RN3 (Stichocorys wolffii Interval Zone) are divided into subzones based on the last occurrences of Calocycletta costata and Carpocanopsis cingulata, and the first occurrence of Liriospyris stauropora, respectively. Designation of these zones/subzones improved the biostratigraphic resolution (average of one subzone/zone per 1.2 million years). A new species, Pterocanium maldives, is described. Comparison of results from Site 714 with those of previous studies from the tropical Pacific Ocean enables identification of synchroneity and diachroneity of radiolarian datum events between the tropical Indian and Pacific Oceans. Of 64 radiolarian bioevents in the tropical Indian Ocean from the early to late Miocene, 22 were apparently synchronous or nearly synchronous. These radiolarian events provide useful biohorizons for stratigraphic correlation and age determination of lower Neogene sequences from the tropical Indian to Pacific Oceans. Patterns of synchroneity and diachroneity of first occurrences were different near the middle/late Miocene boundary. Below this transition, the first occurrences were synchronous from the tropical Indian and Pacific Oceans. Above this transition, they were earlier in the tropical Indian Ocean than in the tropical Pacific Ocean. Changes in these patterns might to have been influenced by the effective closure of the Indo–Pacific seaway. In contrast, the large majority of last occurrences showed synchroneity from the tropical Indian and Pacific Oceans during the early to late Miocene. This suggests that last occurrences during the early to late Miocene were affected by global climatic or oceanographic events.