Middle to Late Miocene paleoceanographic evolution of the south-eastern Indian Ocean (ODP Site 752) inferred from nannofossil assemblages

Abstract. Understanding how the Earth’s system behaves under climate forcing conditions is critical for predicting how future climate change scenarios may affect the planet. In this regard, the Miocene, with an atmospheric temperature ~4 °C higher than modern and a near-modern ocean configuration, stands out as a potential analogue for future climate projections. Our current understanding of the Miocene derives principally from the Atlantic and Pacific Oceans, whereas the Indian Ocean remains understudied. Nevertheless, different studies performed in the equatorial Indian Ocean have revealed that the multiple climate shifts that occurred during the Miocene modified both the atmospheric and oceanic processes in this basin (e.g., changes in westerlies position, nutrient flux and productivity). In the present study, we investigate how changes in the climate conditions affected the surface ocean system at Ocean Drilling Program (ODP) Site 752, located in the southeastern Indian Ocean, across the Middle to Late Miocene. For this purpose, we present a new quantitative dataset of calcareous nannofossil assemblages, in conjunction with already existing multi-proxy data.

Our results indicate that the warm atmospheric conditions reached during the Miocene Climatic Optimum (MCO), starting at ~16.9 Ma, caused an intensification of the seasonal signal at ODP Site 752 that lasted until 15.32 Ma. After this period, we detected a shift in the calcareous nannofossil assemblage towards species characteristic of low-nutrient and warm-water conditions (e.g., Reticulofenestra pseudoumbilicus, Reticulofenestra haqii). Nevertheless, with the progression towards the Late Miocene Cooling, which started at ~7 Ma, and the increased influence of the cooler and more productive Southern Ocean waters, the calcareous nannofossil assemblage evolved into a community characterised by relatively high-nutrient and cold-water taxa (e.g., Coccolithus pelagicus, Calcidiscus leptoporus). By 10.67 Ma, we recorded a reversion in the nannoplankton assemblage to species typical of low-nutrient and warm-water conditions that lasted until 9.88 Ma. The comparison with available eNd(t) records for Site 752 and 707 (located in the equatorial Indian Ocean) revealed that this change in the assemblage composition responded to a strengthening of the Pacific Ocean water influx occurring at this time. High-nutrient and cold-water conditions were reestablished after 9.88 Ma, lasting until the end of the studied record at ~7.34 Ma. Our evaluation of the primary producer community from a multi-proxy-based perspective revealed a progressive evolution of the surface ocean conditions in the southeastern Indian Ocean region across the Middle to Late Miocene for the first time. Furthermore, the strong correlation with existing proxy data for global climate records suggests that large-scale climate events are the main factors driving the changes in the primary producer community and surface ocean conditions at the Broken Ridge during the Middle to Late Miocene. However, certain regional processes (e.g., strengthening of the Pacific Ocean influx) may also occasionally influence surface ocean conditions. Moreover, this multi-proxy approach allowed us to better understand the Indian Ocean–Southern Ocean interaction, and more specifically, the nutrient transport efficiency between these two ocean basins during a climate change scenario. Specifically, our multi-proxy-based dataset revealed that the Antarctic divergence region experienced a northward migration after the MCO, leading to a deep reorganisation of nutrient transport across the surface southeastern Indian Ocean.