Paleoceanographic changes in the Northeast Indian Ocean during middle Miocene inferred from carbon and oxygen isotopes of foraminiferal fossil shells

Abstract

The middle Miocene transition witnessed rapid fluctuations in oceanographic and climatic systems. However, very limited work has been attempted in Miocene sediments of the Northern Indian Ocean to identify these changes. Here we present the first detailed δ13C and δ18O records of surface and deep dwelling foraminifera from ODP site 758 in the Northeast Indian Ocean, located atop the Ninetyeast Ridge. Carbon and oxygen isotopic data on Globigerinoides quadrilobatus (planktonic) and Cibicidoides wuellerstorfi (benthic foraminifera) indicated various global and local successions of oceanographic and climatic importance. The climatic events recorded at ODP 758 are; (a) Mid Miocene climatic Optimum (17–15 Ma), (b) Monterey excursion (17–14 Ma), (c) East Antarctica Ice sheet formation (13.8 Ma), (d) Initiation of Indian Summer Monsoon with waning of Antarctica Ice sheet (12.3–10.4 Ma), and (e) evidence of a cooling event (10.2–9.6Ma).Our benthic foraminifera δ18O record showed almost identical deep water evolution to that of the Indo Pacific for 13.9–11.5 Ma interval. A significant decrease in δ13Cben at ~10.2 Ma was indicative of a complete re-adjustment of the carbon cycle after the Monterey excursion and export of the North Atlantic Deep Water (NADW) towards the southern Ocean to give rise to the Circumpolar Deep Water (CDW), with a δ13C value of ~0.4‰. The formation of the NADW had given rise to modern deep water like conditions at ~10 Ma ago. A comparison of our record with previously published isotopic records from other regions suggested a good correlation between the South Pacific, Indo Pacific and Northeast Indian Ocean during the studied time interval. It appears that a strong westerly Indian Ocean current (MIOjet) may have linked these Oceans from 14 to 9 Ma.