A pronounced deep water cooling in the Indian Ocean at ∼ 3.3–2.3 Ma linked to a major increase in the Antarctic ice volume

Abstract

This study investigates changes in the bottom water circulation of the Indian Ocean since 6.5 Ma, based on benthic foraminiferal relative abundances and their stable isotope ratios from Ocean Drilling Program (ODP) Sites 709 and 758. A significant shift, from dominant North Indian Deep Water (NIDW) to Antarctic Bottom Water (AABW) circulation, is documented at ∼3.3 Ma. Principal Component Analysis (PCA) reveals two major benthic foraminiferal assemblages (PCA1 and PCA2). PCA2 is dominated by Nuttallides umbonifera, Globocassidulina subglobosa, and Epistominella exigua, representing cold and well‑oxygenated bottom water conditions between 3.3 and 2.3 Ma. The increased δ¹⁸O values of Cibicides wuellerstorfi and high relative abundance of N. umbonifera between ∼3.3 and 2.3 Ma at both sites, indicate an increased influence of AABW in the Indian Ocean due to a major increase in Antarctic ice volume. PCA1 is dominated by Uvigerina proboscidea, Fissurina spp., and miliolids, indicating oxygen-poor bottom waters and high surface productivity between 1.7 and 0.5 Ma associated with an influence of NIDW. The high relative abundance of U. proboscidea and low relative abundance of E. exigua signify low seasonality and sustained flux of organic matter with a signature of strong South Equatorial Counter Current formed by the seasonal reversal of winds during 1.7–0.5 Ma at Site 709.