Impact of Western Pacific Warm Pool variability on regional and global climate evolution during the late Miocene and early Pliocene
Final Report Abstract
As the warmest water body on the planet, the Indo Pacific Warm Pool (IPWP) strongly influences the dynamics of the El Niño Southern Oscillation and the Asian-Australian monsoon system, affecting planetary-scale atmospheric circulation, atmospheric heating, and tropical hydrology. To date, there is considerable uncertainty regarding the response of the tropical Pacific climate to rising greenhouse gas concentrations and sea level due to our limited understanding of the IPWP past variability and conflicting results from data and model simulations. During IODP Expedition 363, an extended (sedimentation rate: ~6 cm/kyr), undisturbed upper Miocene to lower Pliocene hemipelagic succession was retrieved for the first time at the southwestern edge of the present day IPWP (Site U1482, 15°3.32ʹS, 120°26.10ʹE, water depth: 1466 m). This carbonateand clay-rich sequence provides an ideal archive to closely track the evolution of the IPWP and the Australian Monsoon and to explore climate-carbon cycle dynamics under different mean-state background conditions on a warmer-than-modern Earth, thus, helping to guide models and constrain predictions of climate change and sensitivity. Using a combination of high-resolution ocean temperatures and d18Oseawater reconstructions (based on stable isotope and Mg/Ca analyses of mainly unaltered, glassy foraminifers) and X-ray fluorescence (XRF) scanner derived monsoonal runoff records, we aimed to 1) monitor variations in the spatial extent of the IPWP in relation to high-latitude climate change on orbital timescales; 2) investigate the long-term evolution of the Australian Monsoon and its linkages with other monsoonal subsystems; 3) assess changes in the zonal and meridional temperature gradients under differing mean-state background conditions and 4) evaluate the role of intermediate waters as a buffer of high-latitude climate variations. Building on the shipboard stratigraphy and our revision of the sediment splice, we developed a new orbitally-tuned chronology over the interval 8.5 to 4.2 Ma at Site U1482, based on correlation of our high-resolution benthic foraminiferal isotope and XRF scanning elemental records to the latest orbital solution. XRF scanning derived runoff proxy records at Site U1482 reveal a strengthening of the precessionally modulated wet austral summer monsoon after ~7.0 Ma, likely associated with regional aridification, which coincided with the onset of a prolonged global cooling spell lasting until ~5.5 Ma. Synchronous changes in monsoonal regime in the Northern and Southern Hemispheres after ~7 Ma indicate a close coupling of the Australian and Asian monsoon subsystems following global cooling. We speculate that the Inter-Tropical Convergence Zone shifted further south, as the Northern Hemisphere cooled, resulting in decreased influence of tropical convection over southeastern Asia and intensification of the dry boreal winter monsoon, while the wet austral summer monsoon intensified over northwestern Australia. Transient global cooling events through the latest Miocene and earliest Pliocene were associated with marked decreases in the intensity of the summer monsoon in both hemispheres, demonstrating dynamic linkages between high- and low-latitude climate change. Our records from Site U1482 additionally reveal that the early Pliocene warm period was characterized by a major intensification of the Australian Monsoon. A highlight of this project was the outstanding match of XRF scanner derived data with local precessional insolation, which sheds new light on the driving mechanisms of monsoonal variability. Our research will ultimately be integrated with emerging new results from IODP Expedition 363 and will complement the overarching goal of several recent IODP expeditions to reconstruct the Neogene evolution of the IPWP and the variability of the Indo-Asian-Australian monsoon system over multiple timescales.
