Stable isotopic study of northeast Indian Ocean during Miocene: An evidence of surface and deep water changes.

The middle Miocene transition witnessed rapid changes in oceanographic and climatic systems. It has been suggested that cooling of southern latitude surface waters during the middle Miocene increased production of deep and intermediate masses and enhanced vertical temperature stratification.

Here we present δ¹³C and δ¹⁸O records from surface and deep dwelling foraminifera from ODP site 758 retrieved from the Ninetyeast Ridge (Lat. 5o23’N, Long. 90o21’E; water depth 2925 m). Chronological framework of the core is based on chemostratigraphy and biostratigraphy. Isotopic studies were carried out on two species Globigerinoides quadrilobatus (planktic) and Cibicides wullerstorffi (benthic).

The δ¹³C values of G. quadrilobatus show two shifts - one at 16.86 Ma and the other at 12.33 Ma. The mean δ¹³C 1.13‰ shift at 16.86 Ma is probably due to the global ‘Montery Excursion’. The less prominent δ¹³C shift at 12.33 Ma is attributed to the regional change in carbon budget. The δ¹⁸O values show two depletion events - one at 17 Ma which is related to mid Miocene Optima and the other at 12.05 Ma which again might be related to local change in surface water characteristics. The enrichment event at 14 Ma is related to east Antarctic ice sheet formation. The carbon Vs oxygen data shows a very evident inverse relationship during 16.84 Ma to 13.03 Ma to suggest a strong coupling between the two isotope tracers.

The average value of δ¹³C of Cibicides wullerstorffi recorded is 1.01‰ with maximum at 13.57Ma 2.61‰ . Our δ¹³C values from ODP 758 are significantly enriched compared to the Holocene Cibicides wullerstorffi values of same site and at nearby locations. We infer that the deep water characteristics and sources of deep water in the North East Indian Ocean were drastically different than the Holocene and last glacial period.