Diachronism in Late Neogene-Quaternary planktic foraminiferal events in Northern and Eastern Indian Ocean: Palaeoceanographic implications

The First and Last occurrences of planktic foraminiferal species in deep sea cores provide useful biostratigraphic datum planes in the Neogene. We examined the Late Neogene-Quaternary planktic foraminiferal biostratigraphies from DSDP holes 219 and 214 (Northern Indian Ocean) and ODP Hole 763A from the Eastern Indian Ocean. Graphic Correlation amongst these three deep sea cores from Northern and Eastern Indian oceans and subsequent integration with magnetostratigraphy resulted in assessing twenty-five Late Neogene- Quaternary planktic foraminiferal events for their usefulness in biostratigraphic correlation. A Composite Standard Reference Section was developed for the Indian Ocean, representing maximum stratigraphic ranges. Numerical age estimates were made for all the planktic foraminiferal events at the three holes. This resulted in a quantitative assessment of the extent of diachronism. We found that out of twenty-five events, six are synchronous, while nine events are synchronous between any two holes and diachronous at the third hole. Ten events were found to be diachronous. Maximum diachronous events showed the extent of diachronism between 0.4 to 1.2 Ma. The maximum extent of diachronism observed was 2.5 Ma. We divided the events into three categories, which include synchronous, fairly synchronous and diachronous events designated respectively as IO-Cat-1, IO-Cat-2, and IO-Cat-3. The primary cause of diachronsim was found to be water mass preferences of individual species. This study provides the first detailed numerical age estimates, the extent of diachronsim, and its palaeoceanographic implications from the Indian Ocean.