Australian Northwest Shelf sedimentation through the Pleistocene: orbital climate variability and sedimentation anomalies

Abstract

Marine sediments from the Australian Northwest Shelf (NWS) are ideal archives for investigating past changes in the regional climate, oceanography, and shelf sediments. Important features of this region are the Australian monsoon, significant offshore dust fluxes, the Holloway/Leeuwin Current (HC/LC) extending southwards from the Indonesian Throughflow (ITF), and an extensive carbonate ramp system. This thesis investigates these features using Plio-Pleistocene sediment samples from International Ocean Discovery Program (IODP) Site U1464 in three research chapters. Firstly, a new, orbitally-tuned chronology for Site U1464 was established based on biostratigraphy, eccentricity and obliquity tunings, and dust-flux proxies for the Early Pleistocene were generated based on new, high-resolution geochemical and environmental magnetic records. These dust-flux proxies revealed strong obliquity variabilities in Northwest Australian dust fluxes, where the 41 kyr periodicity is attributed to linear responses to the East Asian winter monsoon and/or the summer inter-tropical insolation gradient (SITIG), while the 54 kyr variability may be a non-linear response to obliquity amplitude modulation via the SITIG effect on cross-equatorial flows. This study observed a dramatic lithological transition at Site U1464, where terrigenous element concentrations and environmental magnetic signals dropped at ~1.7 Ma while carbonate concentrations increased synchronously. This lithological transition was coincident with marked changes in geophysical data and foraminiferal assemblage, and a rise in aridity proxy records at Site U1464. The geophysical and foraminiferal data suggest that along-shore winnowing intensified by the ITF/HC during ~1.7-1.4 Ma and offshore winnowing became an important transport process on the NWS during subsequent 1.4-0.8 Ma, while the aridity proxy records are indicative of aridification at ~1.6 Ma. These geochemical, geophysical and microfaunal observations are best explained by a major transition in regional climate and oceanography. Specifically, the records strongly suggest that the modern Australian aridification was initiated at ~1.6 Ma and might not be driven by a weakened ITF/HC as proposed by previous studies, but rather by establishment of modern Walker and Hadley circulations. Finally, this work also reveals dolomites on and within planktonic foraminiferal tests from IODP Site U1464. Fossil planktonic foraminifera are important climatic archives, from which ancient seawater chemistry and climate variability can be revealed (e.g., based on Mg/Ca, δ18O and δ13C records). The validity of applying such methods to NWS sediment samples may therefore be compromised by diagenetic dolomite formation. This is investigated using new scanning electron microscopy, X-ray diffraction, stable isotopes (C, O), Mg/Ca, and rare earth element data combined with shipboard porewater geochemistry. Results suggest that foraminiferal dolomite formation at Site U1464 was likely associated with early diagenesis at the seafloor and post-depositionally, as well as microbial and aging processes. Shallow marine environments may be more prone to dolomite and proto-dolomite formation, compared to typical deeper pelagic environments, due to higher fluxes of organic matter to the sea floor. Show more