Impact of the Indonesian Throughflow on northwestern Australian biochronology during the Pliocene (IDEAL).

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International Ocean Discovery Program (IODP) Expedition 356 (August-September 2015) drilled a transect across 10° latitude of seven shelf and upper slope sites (Sites U1458-U1464) off Western Australia from the Perth Basin, through the Northern Carnarvon Basin, to the Roebuck Basin with the RV Joides Resolution. One of the main objectives was documenting the evolution of the Indonesian Throughflow (ITF) since the early Pliocene, a critical component of global thermohaline circulation and a driver of the southward-flowing Leeuwin Current. This in turn has influenced the development of aridity in Australia and the onset of the Australian monsoon. Despite the modern shallow water depth of the sites, significant subsidence during the late Miocene created an upper-bathyal setting, so that open-marine conditions prevailed during the Pliocene. This setting, which is located directly in the outflow of the ITF, allowed accumulation of a unique sequence of sediments constituting a high-resolution record of ITF changes since the early Pliocene. Previous studies from the Maritime Continent and from further offshore western Australia have shown that significant changes in the ITF occurred during the Pliocene. However, high-resolution records from directly within the outflow, which have the potential to directly link these changes to development of aridity and the monsoon across Australia, were lacking prior to Expedition 356. This project aims to improve the biostratigraphy in the eastern Indian Ocean by testing the following hypotheses that: a) Biochronology off the northwest coast of Australia remained similar to the equatorial Pacific until 3.3 Ma; b) The Pliocene development of the biogeography of shallow- and deep-dwelling planktonic foraminifera was distinct due to restriction of the Indonesian Throughflow. Creating a reliable reference record requires a well-constrained age model. It is therefore planned to create a benthic foraminiferal stable oxygen isotope record with orbital-scale resolution that will be tuned to the global benthic d18O stack (LR04). This age model will then be used to calibrate the planktonic foraminiferal biostratigraphy in the eastern Indian Ocean. Typically, Pliocene bio-datums are either from the Atlantic or the Pacific and often have differences of up to several 100 kyr between them. IODP Site U1463 was selected for this project because its position is ideally located directly within the outflow of the ITF, and meanwhile far enough offshore to be minimally impacted by terrigenous impact so that planktonic foraminifera are abundantly present.