Late Neogene nannofossil assemblages as tracers of ocean circulation and paleoproductivity over the NW Australian shelf

Abstract. Late Miocene to Pliocene sediments from the NW Australian shelf provide unique records of paleoclimatic variations under warmer-than-present conditions. During the period from 6–3.5 million years ago (Ma), the area was dominated by warm, tropical waters supplied by an intensified, uninterrupted Indonesian Throughflow and characterised by prevailing humid conditions and increased precipitation. Despite the available information regarding the general paleoclimatic conditions, little is known about the concurrent regional ocean circulation patterns and the relative intensity of seasonally flowing boundary currents, such as the Leeuwin Current. Recent investigations of sediments recovered from the shelf during International Ocean Discovery Program (IODP) Expedition 356, have produced a number of well-dated time-series that cover the late Neogene and can therefore assist with more detailed reconstructions. In this study, we investigate two astronomically-tuned calcareous nannofossil records from IODP Sites U1463 and U1464 that can help us trace and understand long-term changes in ocean circulation and seasonality, as well as their effect on nutrient replenishment in the upper photic zone. By looking into shifts in the dominant species within the nannofossil assemblage, and comparing them with paleotemperature gradients between the NW Australian shelf and the eastern Indian Ocean, we identify the main periods of change in stratification and nutrient availability that occurred over the continental shelf. Our results indicate a significant change in ecological and oceanographic regime that occurred across the Miocene to Pliocene boundary (5.4–5.2 Ma), which can be attributed to an increase in seasonality and an overall intensification of the upper water column mixing over the shelf. Major changes in the nannofossil assemblages that reflect broader-scale processes and evolutionary events, such as the extinction of Sphenolithus spp. (~3.54 Ma) and the termination of the late Miocene to early Pliocene biogenic bloom in the eastern Indian Ocean (4.6–4.4 Ma), occurred long after this regional regime shift.