Late Cenozoic Qaidam Basin Climatic And Environmental Evolution And Forcing Mechanisms

The evolution of Asian monsoon and central Asian aridification are the products of Tibetan Plateau uplift,global cooling and retreat of the Tethys sea.However,how these factors contribute to the formation of central Asian arid environment and the evolution of Asian monsoon remains unclear.Generating reliable late Cenozoic climatic and environmental records from central Asia is not only helpful to clarify these long-term climate change,but also is essential to understand orbital-timescale Asian climate variations.A basic framework of late Cenozoic climatic and environmental variations in East Asia was constructed based on loess-red clay sequences from Chinese Loess Plateau.However,due to the uncertainty of climate proxies and the lack of high-resolution long-term palaeoclimate records with precise dating,late Cenozoic Asian climatic and environmental changes at tectonic and orbital timescales and their forcing mechanisms before Quaternary are still controversial.In this research,paleoenvironment reconstruction was conducted at three locations in the Qaidam Basin,including the Dahonggou section,the Huaitoutala section and the Huatugou section.The age models for the above sections have been established based on the paleomagnetism and paleontological fossils.Detailed analyses of magnetic and isotope geochemistry proxies were performed to investigate the late Cenozoic Asian monsoon and central Asian aridification evolution on different time scales.Furthermore,we tried to distinguish different impacts of Tibetan Plateau uplift and/or global cooling on the evolution of Asian monsoon and central Asian aridification and clarified orbital-timescale variations of Asian monsoon-arid environment and their link with solar insolation and the internal variations of the Earth's climate system such as ice volume.I summarize the main findings as below:?1?We reported Miocene-early Pliocene?²0-5 Ma?paleovegetation history from the fluvio-lacustrine sediments in the Dahonggou section of the Qaidam Basin.These results reveal a dominant C3 plants during²0-5 Ma and no significant C4expansion in the late Miocene in the NE Tibetan Plateau,in contrast with the documented C4 expansion in the late Miocene on Chinese Loess Plateau.We infer that no significant C4 expansion in the late Miocene in the Qaidam basin is related to the regional temperature drop due to tectonic uplift of Tibetan Plateau.Our study provides paleoecological evidence for the late Miocene uplift of Tibetan Plateau,consistent with tectonic evidence.?2?We established an East Asian summer monsoon?EASM?record during¹0-6Ma from the lacustrine sediments in the Huaitoutala section of the Qaidam Basin.The Qaidam Basin records demonstrate that intensified precipitation and wetter climate occurred during⁸.5-7 Ma.The phase of climate wetting is also recorded in Zhuanglang and Qin'an sites on Chinese Loess Plateau.We attribute the intensified wetting climate to the northwestern migration of EASM rain belt due to the growth of the northeastern Tibet during the late Miocene.This inference is consistent with the above ecological evidence and further confirms the late Miocene uplift of Tibetan Plateau.?3?We reported a phase of intensified aridity at³.3 Ma and no central Asian drying at the Pliocene-Quaternary boundary in the Qaidam Basin.This is consistent with the geological evidence from the Tarim Basin.The late Pliocene drying may be attributed to the Indian Ocean cooling caused by gradual closure of the Indonesian Seaway.The investigation suggests tropical oceans can affect Asian environmental evolution in middle latitudes.?4?The EASM record appears to exhibit a dominant 100-kyr periodicity during the late Miocene?⁸.5-7 Ma?,similar to the EASM records during the late Quaternary.Because partial or ephemeral ice existed in the Northern Hemisphere during the late Miocene,we attribute the 100-kyr cycles to CO₂ and Southern Hemisphere insolation-driven Antarctic ice sheet forcing.This indicates a>6-million year earlier onset of the dominant 100-kyr Asian monsoon and improves the understanding of EASM variations at orbital timescale and its forcing mechanism.?5?The late Pliocene?³.6-3.1 Ma?chemical weathering intensity variations in the Qaidam Basin indicated by magnetic susceptibility record appear to exhibit dominant 20-kyr precession cycles,however,the aridification variations indicated by halite content record appear to exhibit dominant 100-kyr eccentricity cycles.This result suggests that different climatic aspects have different sensitivity to orbital forcing and aridification is more sensitive to eccentricity forcing,but precipitation is more sensitive to precession forcing.These results provide new data for understanding the evolution of East Asian monsoon and the formation of central Asian arid environment,and further indicate that the Tibetan Plateau uplift may play a crucial role in the climate and environment evolution of the Qaidam Basin.The closure of Indonesian seaway and the Antarctic ice sheet variations may also play an important role in the climate and environment change of the Qaidam Basin,however,these two forcing mechanisms did not get enough attention before.