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

Abstract

Abstract. Late Miocene to Pliocene sediments from the NW Australian shelf, drilled by the International Ocean Discovery Program (IODP) Expedition 356, provide unique records of paleoclimatic variations under warmer-than-present conditions. During the period from 6 to 3.5 Ma, the area was dominated by warm, tropical waters supplied by an intensified, uninterrupted Indonesian throughflow and was characterised by prevailing humid conditions, including increased precipitation and river runoff. Despite the available information regarding the general paleoclimatic conditions, little is known about the concurrent regional ocean circulation patterns and the relative strength of seasonally flowing boundary currents, such as the Leeuwin Current. In this study, we investigate two astronomically tuned calcareous nannofossil time series from IODP Sites U1463 and U1464 to track long-term changes in ocean circulation and water column stratification, which influences the availability of nutrients in the upper photic zone and is considered to be a primary control on the (paleo)productivity of marine phytoplankton. By documenting shifts in the dominant species within the nannofossil assemblages and comparing these to paleotemperature gradients between the NW Australian shelf and the eastern Indian Ocean, we identify a significant change in the ecological and oceanographic regime that occurred across the Miocene–Pliocene boundary (5.4–5.2 Ma), which can be attributed to an overall intensification of the upper water column mixing over the shelf. Significant changes in nannofossil abundance and species composition that reflect broader-scale processes and evolutionary events, such as the termination of the late Miocene to early Pliocene biogenic bloom in the eastern Indian Ocean (4.6–4.4 Ma) and the extinction of Sphenolithus spp. (∼3.54 Ma), occurred long after this regional regime shift.