Orbital-scale Asian Monsoon Evolution And Response To Global Change During The Pliocene-Pleistocene Climate Transition

The mid-Pliocene wasthe last period in Earth’s history when global temperatures were significantly warmer and global ice volume was smaller than today.No other warm period in Earth’s history had a unique combination of atmospheric CO₂concentrations,paleogeography and paleobiological conditions similar to those of modern times as the Pliocene warm period.These similarities make the study of climate change characteristics during the MPWP period very useful for understanding the future evolution of the Earth system as anthropogenic CO₂ emissions continue to push the Earth into a“mid-Pliocene warm period”state.The late Pliocene transition of the Earth’s climate occurred rapidly,with atmospheric CO₂ concentrations declining to pre-industrial levels and the establishment of a permanent Arctic ice sheet at～2.7 Ma.The mechanism of the late Pliocene climate transition and the origin of Northern Hemisphere glaciers remains an ongoing topic of discussion.The Asian monsoon is an important part of the global climate system,and its formation and evolution have profoundly influenced the changes of the environment and landscape as well as the production and life of billions of people in Asia.At present,there are differences in the research results of different scholars regarding the evolutionary process of Asian monsoon and its changes at key time points.Sedimentary records from the northeastern basin of the Tibetan Plateau show that the Miocene-Pliocene East Asian summer monsoon（EASM）rainfall shows a monotonic eccentricity domination.In contrast,the Pleistocene record shows a more complex orbital drive.Little is known about when and why such variations in the dominance cycle occurred.In addition,the large-scale expansion of the northern hemisphere ice sheet in the late Pliocene introduced unique high-latitude glacial dynamics to the Asian monsoon,and thus the evolution of the Asian monsoon during the late Pliocene-early Pleistocene transition is important for understanding the dynamics of the monsoon system,especially the response of the monsoon to high-latitude forcing.Meanwhile,compared to the widely studied East Asian monsoon,the evolution of the pre-Quaternary orbital scale South Asian monsoon is still inadequate.Based on the Pliocene to early Pleistocene lake sediments from the Sanmenxia Basin in northern China and the Yuanmou Basin in southwest China,this thesis investigates the evolution of the East Asianand South Asian summer monsoon from the mid to late Pliocene to the early Pleistocene at the orbital scale and analyzes their driving mechanisms through petrostratigraphy,magnetostratigraphy,cyclostratigraphy and paleoclimate proxy analysis.In order to investigate the response of the Asian monsoon to global climate change and the influence of oceanic processes on the monsoon and global climate change,published orbital-scale global oceanic sea surface temperature data since the Pliocene were also collected and analyzed in this study.From the above study,the following new insights were obtained:1)An astronomical age scale was established for the Huangdigou section of the Sanmenxia Basin,and the evolution of Sanmen basinwas found to be dominated by～100 kyr eccentricity cycles to be driven by a combination of 100 kyr eccentricity and 41 kyr obliquity cycles during the late Pliocene.By collecting XRF rock element content data and testing and analyzing paleomagnetic samples from the 81 m-thick Huangdigou section of the Sanmen Formation in the Sanmenxia Basin,North China,an astronomical dating framework for the 3.85-2.2 Ma Huangdigou section was established using a combination of biostratigraphy,magnetostratigraphy and cyclostratigraphy.The results show that the evolution of Sanmen paleolake in the mid-Pliocene（3.85-2.75 Ma）was mainly dominated by orbital eccentricity,while a major shift from eccentricity-dominated to obliquity-driven hydroclimate occurred during the late Pliocene-Pleistocene（2.75-2.6Ma）,which is also related to the increase of regional climate stochasticity shown in the recurrence analysis.Combined with the results of previous cyclostratigraphic studies on the lacustrine sedimentary record at the northeastern margin of the Tibetan Plateau,we suggest that under the dominant control of the unipolar ice sheet,the East Asian summer monsoon evolution was dominated by eccentricity;whereas,after the emergence of the bipolar ice sheet,the East Asian summer monsoon response shifted to a joint dominance of eccentricity and obliquity cycles.Thus,the results emphasize that global boundary conditions and local low-latitude insolation forcing have equally important effects on the East Asian monsoon.2)A significant enhancement of summer rainfall in East Asia was found in the Sanmen paleolake record during the late Pliocene.The continuous lacustrine sedimentary record of the Huangdigousection in the Sanmenxia Basin shows a significant enhancement of regional hydrodynamic conditions reflected by titanium（Ti）,vanadium/chromium（V/Cr）,vanadium/（V+Ni）[V/（V+Ni）]and grain size indicators around the intensification of Northern Hemisphere glaciation（i NHG）at～2.75 Ma,which may reveal a significant increase in summer monsoon rainfall in East Asia.Our study of sea surface temperature gradients during this period shows that the Atlantic and Pacific meridional and latitudinal sea surface temperature gradients increased significantly during the same period,which may be due to the coupled ocean-atmosphere that enhanced the global Hadley and Walker Circulation and thus strengthened the East Asian summer monsoon.3)An astronomical age scale of the Gantang section in the Yuanmou Basin was established,and it was found that the evolution of the Yuanmou basin was mainly driven by 405 kyr and～100 kyr eccentricity cycles from the mid-Pliocene to the early Pleistocene.By collecting XRF rock element content data and testing and analyzing paleomagnetic samples from the 90 m-thick Gantang section of the Shagou and Yuanma Formations in the Yuanmou Basin,Yunnan,China,we established a high-resolution astronomical age scale of 4.2-2.0 Ma for the Gantang section using a combination of petrostratigraphy,biostratigraphy,magnetostratigraphy and cyclostratigraphy.Combining the marine record of the Arabian Sea and the terrestrial record of the Zhaotong Basin in Yunnan,we suggest that the South Asian summer monsoon were stronger during the mid-Pliocene warm period,while the intensity of the late Pliocene monsoon weakened significantly,and the variation of its intensity was closely related to the interhemispheric pressure gradient caused by temperature changes at high latitudes in the Southern Hemisphere.In addition,the weakening of the low-latitude atmospheric circulation in the Indian Ocean may also be an important reason for the significant weakening of the South Asian summer monsoon at 2.9-2.75 Ma.Our study shows that the evolution of the orbital-scale South Asian summer monsoon is mainly controlled by the low-latitude solar radiation/solar radiation gradient,which is characterized by a dominant eccentricity cycle,while the significant obliquity signal in the 3.3-2.9 Ma geochemical record should be attributed to the expansion of the West Antarctic ice sheet at this time.In addition our comparison of orbital-scale East Asian summer monsoon records shows that the South Asian and East Asian monsoons have different responses to astronomical driving,which may be related to the different latitudes at which the two monsoon subsystems are located and the difference in sensitivity to high-latitude processes in the Northern Hemisphere.4)It is revealed that ocean temperature has played a key role in global climate change over the past 4 million years,and that ocean processes have in turn influenced the evolution of the Asian monsoon through the regulation of atmospheric CO₂ and high-latitude ice sheet.To investigate the role of the ocean in monsoon evolution and global climate change,this study synthesizes orbital-scale sea surface temperature（SST）data from the North Atlantic,North Pacific,South China Sea,and Southern Ocean and finds that the evolution of SST since 4 Ma shows dominant orbital eccentricity and obliquity cycles.We calculated the vertical（thermal）stratification of the ocean from the vertical temperature gradient of the ocean and inferred that the increase in stratification drives the changes in cryosphere and atmospheric CO₂ concentration,thus supporting the main climate changes since～4 Ma.The results indicate that changes in the equatorial to polar SST gradient were critical to the expansion of the Northern Hemisphere ice sheet at～2.7 Ma,and demonstrate that increased high-latitude ocean stratification at～2.7 Ma and～1 Ma played a key role in changes in atmospheric CO₂ concentrations,driving and sustaining global climate change at that time.And from～1 Ma to the present,the Southern Ocean sea surface temperature may have become an important driver of global carbon cycle and ice volume changes.The findings indicate a long-term trend of increasing carbon emissions from the ocean to the atmosphere nowadays,which will likely be the long-term background for the superposition of natural and anthropogenic carbon emissions in the future.