Middle to Late Miocene paleoceanographic evolution of the southern Indian Ocean (ODP Site 752) inferred from nannofossil assemblages and carbon isotopes

The Middle to Late Miocene constitutes a critical time interval on a global scale. The continental reorganization that occurred during this time caused the establishment of a near-modern monsoonal wind system in the Indian Ocean (IO). Furthermore, this period is characterised by the succession of climatic scenarios linked to the Middle Miocene Climatic Transition (MMCT) and the subsequent northward shift of the Westerlies region during the Late Miocene. How these processes affected and interactedi with the surface ocean dynamics in the southern IO remains poorly understood. In this regard, Ocean Drilling Program (ODP) Site 752, located on the west flank of Broken Ridge (30° 53.475ˈS/93° 34.652ˈE), constitutes a key location to investigate how the aforementioned processes interacted with the surface current in the eastern sector of the southern IO across the Middle to Late Miocene.

Changes in the nannofossil assemblage between 7.31 and 16.06 Ma at Site 752 were evaluated to ascertain variations in the surface ocean conditions. A total of 122 samples (temporal resolution of ~60 kyr) were analysed for this purpose. The clustering ordination method UPGMA (Bray-Curtis) revealed a total of 5 clusters (Cluster 1-5). Cluster 5 was additionally divided into two sub-clusters (Cluster 5a-5b). The oceanographic conditions were inferred based on the abundance of the main nannofossil species constituting the clusters: Reticulofenestra minuta, Calcidiscus leptoporus, Coccolithus pelagicus, Reticulofenestra haqii, Reticulofenestra producta, Reticulofenestra pseudoumbilicus, and Reticulofenestra perplexa. Subsequently, the assemblage data were compared with a set of global climatic and geochemical data to ascertain the effect of global processes on the regional oceanographic configuration across the Middle to Late Miocene.

Total organic carbon (TOC) and Total Inorganic Carbon (TIC) were measured to ascertain variation in the carbon flux to the ocean floor. Additionally, bulk, organic, and benthic foraminiferal δ¹³C analyses were performed in order to track productivity changes and variations in the nutrient cycle during the studied time interval in the southern IO and compared with the observed variations in the nannofossil assemblage. Benthic δ¹³C was measured on two foraminifera species (Lobatula wuellerstorfi and Cibicidoides mundulus).

Our data indicate that the surface water of the southern IO was characterised by low nutrient availability and high temperature conditions during the Middle Miocene. After the MMCT, the surface ocean experienced an increase in nutrient availability, which was concomitant with a decrease in δ¹⁸O. Maximum surface ocean nutrient conditions were recorded after ~9.9 Ma, coeval with the establishment of the Late Miocene Cooling. A comparison between the nannofossil assemblage data at Site 752 and eNd records available in the literature confirmed the hypothesis that warm water input from the Pacific Ocean into the southern IO increased between ~10.7 – 9.9 Ma. Furthermore, the comparison of the assemblage data against δ¹³C, TOC, and TIC measured at ODP Site 752 allowed us to disentangle the local processes driving changes in the IO surface water conditions.