- 1Department of Earth Sciences, University of Graz, NAWI Graz Geocenter, Graz, Austria (tamara.hechemer@uni-graz.at)
- 2Institute of Geology and Mineralogy, University of Cologne, Köln, Germany
- 3Institute of Geology and Paleontology, University of Münster, Münster, Germany
- 4School of Geography, Geology and the Environment, University of Leicester, Leicester, UK
- 5Department of Environmental Science, School of Earth and Environment, Rowan University, Glassboro, NJ, USA
- 6InProcess Instruments, Bremen, Germany
The Miocene Climatic Optimum (MCO) represents a climate period characterized by lower ice volumes and temperatures that were 3-4°C warmer than today. Indian Ocean Sub-Antarctic Mode Water (SAMW) is primarily formed south of 30°S and is the main return path for deep waters to the surface, migrating and intermixing northwards at Intermediate Water (IW) depths. The modern SAMW transports nutrients into the lower latitudes, strongly impacting mid- and low 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. To better understand trajectories of nutrient fluxes in future climate change scenarios studies in past warm climate analogues of the near future – such as the MCO – are necessary. Thus, we use Ocean Drilling Project (ODP) Site 752, located on Broken Ridge in the southeastern Indian Ocean at a water depth of 1086.3 m, as a key location for understanding changes in IW conditions.
This study aims to reconstruct paleoenvironmental conditions and bottom-water oxygenation at ODP Site 752 during the Middle to Late Miocene (15-8 Ma) using benthic foraminifera assemblages as a proxy for bottom-water-oxygenation and the enhanced Benthic Foraminifera Oxygen Index (eBFOI) for calculating dissolved oxygen content. We combine these assemblage data with Mg/Ca ratios of Cibicidoides wuellerstorfi and Cibicidoides mundulus as a proxy for bottom water temperatures (BWT). For reconstructing sea surface temperatures (SST), and temperatures from the open ocean thermocline, the Mg/Ca data were additionally gathered on the foraminifera species Globigerina bulloides (SST) and Globorotalia menardii (thermocline). We aim to analyze temperature variability through the water column to investigate influxes from cooler water bodies by increasing SAMW intensity and compare our new temperature data with our benthic foraminiferal assemblages. Therefore, we provide novel insights into Late Miocene IW circulation changes and deep water mass variation with the progressive northward shift of the Subantarctic Tropical Front (SAF).
We present a high-resolution record of benthic foraminifera, tracing paleoenvironmental changes in deep water masses in addition to IW variation in the southeastern Indian Ocean. After the MCO, benthic foraminifera assemblages, and respectively the eBFOI indicate a relatively high oxic environment. Starting around 11 Ma, we first detect an increase of dysoxic conditions and deep infaunal foraminifera, e.g. the genus Bolivina spp., with minimal variation in the dissolved oxygen content of the bottom water. Such an assemblage shift is contemporary with increased current winnowing following the northward migration of the SAF. Furthermore, the higher abundance of epiphytic species Cibicidoides wuellerstorfi and Lobatula lobatula, and also Vulvulina pennatula as an elevated epifauna, support an increase in bottom current energy at Broken Ridge from 15 to 11 Ma. Combined, our assemblages and Mg/Ca paleotemperature data suggest that the strengthening of the SAMW and Antarctic Intermediate Water formation in the Late Miocene, since about 11 Ma, resulted in notable changes in bottom water conditions at Broken Ridge, including the increase of current winnowing.
How to cite: Hechemer, T., E. Piller, W., Puentes-Jorge, X., Hauzenberger, C., Petersen, J., Grunert, P., V. Del Gaudio, A., De Vleeschouwer, D., M. Bialik, O., Drury, A. J., Christensen, B., Lyu, J., and Auer, G.: Benthic foraminifera as paleoceanographic tracers in the southeastern Indian Ocean during the Middle to Late Miocene: New insights from ODP Site 752, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15159, https://doi.org/10.5194/egusphere-egu25-15159, 2025.