Late Miocene Environment Variations In The Northeastern Tibetan Plateau Documented By High-resolution Environmental Magnetism Record

The late Miocene was a period of geological history that the continental ice sheets existed only in Antarctica,and there were no permanent ice sheets in the high latitudes of the Northern Hemisphere.Thus,the late Miocene provides an ideal geological window for assessing how climate changes of Asian interior respond to the variations of the solar radiation and internal climate boundary conditions(e.g.ice volume)in the absence of large ice sheets in the Northern Hemisphere.However,most existing paleoclimate data can only describe the long-term evolution trend and average state of the Asian interior.Due to the lack of high-resolution paleoclimate records of the Asian interior,it is difficult to accurately understand the orbital and suborbital scale climate changes.In addition,the uplift of the Tibetan Plateau and global cooling occurred simultaneously during the late Miocene.They are thought to be the two main driving forces of environmental change in Asian interior.Separating the information of regional tectonic activities and global climate change from paleoclimate records has great scientific significance for analyzing the process and mechanism of tectonic climate interaction.The continuous late Miocene sediments with high sedimentation rate and a large number of biological fossils are well preserved in the basin of northeastern Tibetan Plateau,which completely records the history of regional climate change and tectonic uplift.Therefore,the geological archives of northeastern Tibetan Plateau provide a good opportunity to improve our understanding of the orbital and suborbital scale climate changes,and the process of tectonic-climate interaction.Here,we choose the Huitoutala section from Qaidam Basin and Jiarang section from Jianzha Basin as research objects.Based on the paleomagnetic dating and environmental magnetism proxies,we generated two high resolution(~1 ka)dry-wet changes records of the northeastern Tibetan Plateau during the late Miocene(8.8-7.1 Ma and 8.25-7.45 Ma).Based on the spectrum analysis and regional correlation methods,we discussed the orbital scale and suborbital scale climate changes recorded in the basin of northeastern Tibetan Plateau during the late Miocene,as well as the response of regional climate to global climate changes and regional tectonic activities.Finally,the following main conclusions and preliminary understanding are obtained:(1)High-resolution paleomagnetic study confirmed a subchron(C4n.2n?1r)near the top of C4 n.2n(8.108-7.695 Ma)from Huaitoutala section,and the age and duration of the C4 n.2n?1r are 7741.7-7737.9 ka and 3.75±1.45 ka,respectively.These results are consistent with the C4 n.2n?1r that was reported at Ocean Drilling Program Site 1092.Confirmation of the existence of subchron C4 n.2n?1r in a northern hemisphere terrestrial site and first documentation of complete the geomagnetic reversals bounding C4 n.2n?1r suggest that the geomagnetic reversals bounding this subchron left global imprints and that thus they can serve as stratigraphic correlation markers.(2)Based on the paleomagnetic dating and environmental magnetism proxies,we generated two high resolution(~1 ka)dry-wet changes records from Huaitoutala(8.25-7.45 Ma)and Jiarang(8.8-7.1 Ma)section at the northeastern Tibetan Plateau.The results of spectral analysis of environmental magnetism records from Huitoutala and Jiarang section show that the dry-wet variations of northeastern Tibetan Plateau during the late Miocene have significant 400-kyr and 100-kyr cycles.The 20-kyr cycles appeared intermittently and there is also significant 10-kyr(Half-precessional)cycles.The 41-kyr cycles are weak.Based on comparison of the spectrum analysis of the environmental magnetic records,earth's orbital parameters(ETP)and global ice volume,we conclude that the dry-wet changes in the northeastern Tibetan Plateau during the late Miocene were mainly controlled by the eccentricity-modulated lowlatitude summer insolation and periodic fluctuations of Antarctic ice sheets.Combining with the published records from other regions,we infer that,during the absence of large Northern Hemisphere ice sheets period,the effect of the eccentricity-modulated lowlatitude summer insolation and periodic fluctuations of Antarctic ice sheets on orbital climate change is likely a global phenomenon,not just limited to the region in the northeastern Tibetan Plateau.(3)During the low eccentricity period(8.13-8.03 Ma)similar to the future orbital conditions,the 100-kyr cycles recorded by the environmental magnetic record of Huaitoutala section does not weaken.Given no permanent ice sheet existed in the high latitudes of the northern hemisphere and the Antarctic ice sheets changed little,we infer that the strong 100-kyr cycles signal may support the hypothesis that the100-kyr cycles are origin from the free oscillations internal to the earth climate system.The free oscillations cycles come from feedback between the earth's internal climate systems.(4)The results of the cross-wavelet coherence analysis show that the amplitude of the suborbital scale climate changes recorded in the Huaitoutala and Jiarang section has more coherent with ETP on 20-kyr,41-kyr and 100-kyr band than ice volume.These results suggest that the suborbital scale climate changes in the mid-latitude Asian interior during the late Miocene was directly driven by the solar radiation changes,and the influence of ice sheets changes was secondary.These observations are consistent with high-resolution paleoclimate records and climate simulation results in the late Quaternary.(5)Our results suggest that the precipitation of the northeastern Tibetan Plateau was increased during 8.8-7.1 Ma.Based on the published tectonic uplift evidence of the northeastern Tibetan Plateau,Sea surface temperature data,deep-sea benthic foraminifer oxygen isotope data and climate simulation results,we conclude that the increasing precipitation in the northeastern Tibetan Plateau during 8.8-7.1 Ma was caused by the regional tectonic uplift.In addition,we suggest that the significant increase in precipitation over the northeastern Tibetan Plateau does not represent the enhancement of the East Asian summer monsoon,but rather a regional precipitation signal,because the East Asian summer monsoon was weak over the central and eastern parts of the Chinese Loess Plateau and the South China Sea during the late Miocene.These observations suggest that the northeastern Tibetan Plateau climate change has regional characteristics.This work provides high-resolution paleoclimate dataset for exploring the orbital and suborbital climate changes and driving mechanism during the late Miocene,and deepens the understanding of the origin of the 100-kyr cycles and suborbital scale climate changes.In addition,our studies promote the understanding of tectonic-climate interaction in the northeastern Tibetan Plateau during the late Miocene.