Paleoceanographic implications from Middle to Late Miocene benthic foraminifera in the southeastern Indian Ocean (ODP Site 752)

Concerning climatic conditions during the Miocene Climate Optimum (MCO), global temperatures were about 3-4C warmer than modern, and characterized by globally lower ice volume. Indian Ocean Sub-Antarctic Mode Water (SAMW) is primarily formed south of 30S and is the primary return path for deep waters to the surface, migrating and intermixing northwards at Intermediate Water (IW) depths. Today, the SAMW carries access nutrients into the lower latitudes, strongly impacting latitude productivity. During warmer climates, decreasing sea ice may increase nutrient trapping in the Southern Ocean, reducing the nutrient flux through SAMW into the lower latitudes. Thus, the MCO may indicate future climate, nutrient transport, and SAMW formation by exploring differences between cooler (modern) and warmer (MCO) climates. Ocean Drilling Project (ODP) Site 752, located on Broken Ridge in the southeastern Indian Ocean at a water depth of 1086.3 m, is a key location for investigating changes in IW conditions in the Indian Ocean after the MCO and the Middle Miocene Climatic Transition (MMCT). In particular, the reactions to global warming and the reorganization of oceanic and atmospheric circulation following the MCO and MMCT can be detected. This also includes the analysis of SAMW, Antarctic Intermediate Water (AAIW), and Tasman Leakage (TL). The present study aims to reconstruct paleoenvironmental conditions and bottom-water oxygenation at Site 752 during the Middle to Late Miocene (15-8 Myrs). To achieve this, we apply benthic foraminifera assemblages as proxies for bottom water oxygenation, for example, enhanced Benthic Foraminifera Oxygen Index (eBFOI), paleoproductivity, and stable carbon and oxygen isotopes. Initial results in the Middle to Late Miocene show an occurrence of oxic benthic foraminifera at a relatively constant abundance, especially in the early Late Miocene. In addition, in a relatively high oxic environment, an increase of dysoxic conditions occurred during the early Late Miocene, with peaks of abundance in dysoxic and deep infaunal benthic foraminifera. A co-occurrence of infaunal dysoxic and epifaunal oxygen-rich species is accompanied by enhanced current winnowing and an increase of nutrient flux during the Late Miocene (Lyu et al., 2023; DOI: 10.1029/2023PA004761). These data indicate that during the Late Miocene, since approximately 10 Ma, the strengthening of SAMW and AAIW formation resulted in notable changes in bottom-water conditions at Broken Ridge, such as increased current winnowing. The observed changes in IW are potentially linked to the shift of the southern hemisphere westerlies towards the north and the subsequent northward migration of the frontal system in the southern hemisphere around Antarctica after 12 Ma.