Late Pleistocene Age Model for Site U1460, Perth Basin, SW Australian Shelf: Implications for Leeuwin Current History

Abstract

The Leeuwin Current (LC) exerts an important control on modern Australian climate, but its onset is not well defined. The LC is the only southward flowing eastern boundary current. Driven by a pressure gradient set up in the Indonesian Throughflow, its warm waters support reefs to 29°S. It is seasonally controlled south of the Western Cape. Determination of the onset of the LC was a major objective of IODP Expedition 356. Expedition 356 drilling on the western Australian margin provides an opportunity to explore depositional patterns and timing in the region influenced by the current. Site U1460 was drilled in 214.5 m w.d. (Gallagher et al., 2017). Integrated calcareous nannofossil and planktonic foraminiferal biostratigraphy places the upper 86.5 m firmly within the late Pleistocene. However, the glacial- interglacial stratigraphy is uncertain because of the complexity of this shelfal depositional environment. Here we present a likely late Pleistocene stratigraphy based on integrated geochemical and paleontological datasets. A benthic foraminifer (Uvigerina peregrina) stable isotope record provides the foundation for the age model and the data are supported by SST estimates based on Tex86 and alkenones. Our age model places MIS 16 between 104 and 99 m-CSF-A, followed by an expanded MIS 15 section (99 - 50 m-CSF-A). We correlate the interval from 50 - 5 m-CSF-A with MIS 14 to MIS 8, with the largest magnitude glacial events (MIS 12, MIS 6) either condensed or represented as depositional hiatuses. A Recent 14C date at 0.34 mbsf constrains the interval from 2 - 0.5 m-CSF-A to MIS 4-3, which is in good agreement with the base of common Emiliania huxleyi (0.09 Ma) at 2.13 m-CSF-A. Thus MIS 5 is equivalent to the interval from 5 to 2 m-CSF-A. The expanded MIS 15 section follows a geometric change from slope to prograding shelf. It is associated with a shift to infaunal benthic foraminiferal assemblages, abundant sponge spicules, and a reduction in CaCO3%, suggesting that the high MIS 15 sedimentation rate results from increased productivity and/or upwelling. Our stable-isotope-based correlation, combined with paleotemperature reconstructions, points to a modern-like LC in operation at 0.6 Ma (MIS 16 onward), following a change in continental margin geometry. Alternative age-depth models are possible, and are also presented.