Reconstructing Western Australian climate history from marine archives over the past 450 kyr

A meridional transect of Holocene to late Pleistocene sediment archives along the Western Australian Margin provides crucial information to explore the spectrum of natural climate variability and to untangle dynamic processes and feedbacks controlling climate evolution. This information is needed to constrain modeling experiments of past and future climate change. A multitude of proxy reconstructions at drill sites located in the eastern Indian Ocean along the Northwestern Australian Margin is presented, with the main objectives of: (1) evaluating the phase relationship between orbital forcing and Australian monsoonal climate and (2) increasing our knowledge on the coupling between local climate variability in northwest Australia and global climate changes. The variability in sediment discharge from Western Australia provides a powerful tool to monitor changes in the position and intensity of the Australian monsoonal rainbelt. Elemental composition of core-top samples retrieved during R/V Sonne cruise SO257 along the Western Australian Margin closely match elemental signatures of major river catchments and sediment accumulation rates generally increase northward. We integrate high- resolution X-ray fluorescence (XRF) scanning elemental records, carbon and oxygen isotopes and Mg/Ca derived sea surface temperatures from Core SO257–18548 and International Ocean Discovery Program (IODP) Site U1482, situated southwest of the Scott Plateau at the southern edge of the present-day monsoonal belt, from Core SO257–18571, located within the dust-cyclone belt offshore Northwest Cape, and from Core MD01- 2378 and IODP Site U1483, located within the Timor Sea in the easternmost Indian Ocean. The chronology of these sediment successions is based on 14C dating over the last glacial termination and on correlation of the benthic oxygen isotope record to the LR04 stack. We use XRF scanner-derived estimates of terrigenous fluvial discharge and eolian dust combined with foraminiferal isotope and Mg/Ca-derived sea surface temperature data in a suite of sediment cores to reconstruct monsoonal climate evolution over the last four glacial cycles. This latitudinal transect includes the tropical monsoon region of northwestern Australia, and the arid zone where rainfall mainly occurs during landfall of tropical cyclones. Our results suggest that the intensity of Australian monsoon precipitation and the position of the Intertropical Convergence Zone (ITCZ) were strongly affected by changes in Southern Hemisphere temperatures and the interhemispheric temperature gradient. Interglacial and suborbital Southern Hemisphere warm periods were associated with intensification and southward migration of the Australian monsoonal rain belt and heightened cyclone activity, whereas cold periods were characterized by weakened monsoon, extended aridity and intensified trade winds increasing dust fluxes. The intensity of summer monsoon rainfall during glacial terminations is strongly correlated to Antarctic ice core greenhouse gas (CO2 and CH4) concentrations and SST records off northwestern Australia. Distinct pCO2 and methane maxima at the end of Terminations I-IV, in particular the characteristic methane-spike at the end of the last glacial termination (~10–7 ka), are paralleled by massive increases in monsoonal runoff from northwestern Australia. Comparison of our precipitation record with published precipitation records from the northern margin and meteorological center of the Indo-Pacific ITCZ shows that precipitation changed nearly in- phase between the hemispheres on the precession band, possibly linked to expansion and contraction in the latitudinal extent of the Indo-Pacific ITCZ. In contrast, interhemispheric precipitation was nearly in anti-phase on the obliquity band, likely due to latitudinal migration of the Indo-Pacific ITCZ. Our findings imply that tropical hydroclimate cycles were regulated by the orbital forcing but with significantly different response to changes of the ITCZ on precession and obliquity bands. In Chapter 2, the additional oxygen isotopes and Mg/Ca based temperature of planktonic foraminifers data from core top samples collected during R/V Sonne Cruise SO257 in May 2017 present a steep Southward increase in planktonic δ18O, associated with a decrease in SST, which indicates that the southwestern front of the Indo-Pacific Warm Pool is located between 23 and 24°S during austral fall. In Chapter 3 benthic and planktonic carbon isotopes integrated with XRF scanner derived barium, Log(iodine/calcium) and bromine enable us to determine that productivity fluctuations over the last 450 kyr were strongly influenced by Australian summer monsoon variability offshore northwestern Australia (23 and 19 kyr) and were also modulated by glacial-interglacial variability (100 kyr) associated with the intensity of trade winds and sea level–related variations in the intensity of the Indonesian Throughflow (100 kyr).


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