Biostratigraphy and paleoecology of calcareous nannoplankton at Broken Ridge, eastern Indian Ocean, during the Middle to Late Miocene (ODP Site 752)

The Middle to Late Miocene represents an important time interval in the Indian Ocean (IO), due to the reorganization of land masses, the establishment of monsoonal wind system and climatic changes. However, variations in intermediate and surface water dynamics through the IO still remain poorly understood. In this regard, Ocean Drilling Program (ODP) Site 752, located in the western flank of Broken Ridge (30° 53.475ˈS/93° 34.652ˈE), constitutes a key location to understand how the above-mentioned dynamics may have affected the path of oceanic currents within the eastern sector of the IO during the studied Miocene interval.

A first biostratigraphic framework for ODP Site 752 was established during ODP Leg 121. However, the poor preservation of the material for the Middle to Late Miocene sediment interval resulted in only limited biostratigraphic data and consequently low-resolution age-depth model. In this regard, our study presents a newly updated high-resolution biostratigraphic age-depth model for this time period (from ~8 to ~15 Myrs) at Hole 752A, based on fully quantitative nannofossil assemblage analyses. Furthermore, a recently published astronomically tuned age-depth model (Lyu et al., 2023; DOI: 10.1029/2023PA004761), tied to the shipboard biostratigraphic age model, was used in order to validate our new quantitative nannofossil record. The comparison showed a high consistency for the whole record, with the exception of the interval between ~12-13 Myrs, where the age difference between the two respective models is high. We link these discrepancies to the potential variance in age of the recorded bioevents for this interval between basins, as well as the error associated with the mathematical approach of the tuned age model.

Based on our quantitative nannofossil data, we further evaluated overall changes in the nannofossils assemblages at ODP Site 752. The assemblage is dominated by reticulofenestrids (e.g., Reticulofenestra minuta, Reticulofenestra pseudoumbilicus). Other species with a contribution >1%, are Calcidiscus tropicus, Calcidiscus pataecus, Coccolithus pelagicus, Coronocyclus nitescens, Discoaster spp., Helicosphaera spp., Sphenolithus moriformis and Umbilicosphaera jafari. Statistical analyses will be performed on the assemblage to define variations on the nutrient availability of the Broken Ridge surface waters and will be presented afterwards.