Cenozoic Palynoflora Investigations In Honggou Section,qaidam Basin:Constraints On Section Age And Climatic-tectonic Events

The uplift of the Tibetan Plateau exerted a profound influence on the tectonic framework of Eurasian continent,environmental and climatic changes of the central Asia as well as the globe.The Tibetan Plateau has changed the general circulation pattern in Asia for its overall elevation>4000 m.The high elevated Tibet could strengthen the Asian monsoon circulation and aridification of the Asian interior,and also has exerted deep impact both on the process of global cooling and the formation of the Northern Hemisphere ice sheet.These important scientific problems,such as the time and amplitude of uplift of the Tibetan Plateau,the origin and development of the Asian monsoon and aridification of the Asian interior,have been a hot area of research for many years.The Northeastern Margin of the Tibetan Plateau is the frontier of the northeastward expanding of the Tibetan Plateau.As the newest portion of the plateau,the region has undergone wide-spread Cenozoic tectonic deformation.As a result,a number of arcuate basin and range pairs are formed with west-northwest trending in the west and gradually becoming north-south orientation in the east.Among them,the Qaidam Basin is the largest intermontane basin in the northeastern Tibetan Plateau with a maximum thickness of?12,000 m of Cenozoic sediments shed from the surrounding mountains.Thus the sediments preserved in the basin documented valuable information of the tectonic deformation,mountain uplift and environmental changes during the process of basin formation.Here we present the multi-indexes researches,including sporopollen,grain size,carbon content,color and magnetic susceptibility from Honggou section in the northern margin of the Qaidam Basin,which has been well-dated by magnetobiostratigraphy,in order to reconstruct the vegetation sucession and climate change since the late Oligocene and to explore its influence mechanism.Impacts of global climate,regional tectonics,and land-sea redistribution are discussed.The main conclusions are as follows:?1?Because pollen analysis is a key prerequisite for palynoflora investigations,the best extracting methods are discussed firstly.The procedure included crush samples,acid-alkali analysis,heavy liquid flotation and sieving.There are several key steps in this new method.?i?In order to remove organic matter most effectively,samples should be heated until just boiling in³%NaOH solution for not more than 5 min.?ii?KI heavy liquid,with a density of 1.74-1.76,should be used to concentrate the pollen from the dried samples,which could avoid impurities.?iii?Sieving with a 7-?m stainless steel nickel mesh is remarkably better than sieving with a 10-?m nylon mesh.Sieving with a 7-?m stainless steel mesh resulted in the loss of few pollen grains.In contrast,sieving with a 10-?m nylon mesh resulted in loss of many small pollen grains,such as Artemisia and Chenopodiaceae which were commonly found in the arid to semi-arid sediments since Neogene.?2?The pollen concentration is positively correlated with the total organic carbon?TOC?content.For those samples with TOC content above 0.1%,it is 52%in which is abundant of pollen;and there is no pollen in the samples with TOC content below0.06%.Also there are abundant charcoals in the samples with high TOC content.This provides a reference for the effective pollen analysis of the Tertiary red layer in the future.?3?There are 29 out of 240 samples with abundant sporopollen mainly distributed from Xia Ganchaigou Fm.to Shang Youshashan Fm.The palynological record shows that temperate forest with dominance of Ulmus,Betula,Alnus,Cupressaceae,Asteraceae and Chenopodiaceae gradually replaced by conifers-steppe dominated with Abies,Picea,Pinus,Cupressaceae,Artemisia,Chenopodiaceae and Potamogetonaceae at the depth of²200 m.The palynological record of the Honggou section is comparable with the sporepollen records in several Neogene sedimentary basins in the northeast of the Tibetan Plateau,e.g.Tianshui Basin,northwest Qaidam Basin.The occurrence of characteristic pollen taxa in the Honggou section is of great significance to the stratigraphic age:Fupingopollenites most widely distributed in Miocene;and Asteraceae has been abundant since Late Oligocene-Early Miocene.Hence,the palynology record also proved that Honggou section has been deposited since Neogene,which is further proved the magnetobiostratigraphy of the section are reliable?Wang et al.,2017?.?4?The palynological record shows that temperate forest rapidly replaced by conifers-steppe dominated with Abies,Picea,Pinus and Cupressaceae at the depth of²200 m with an age of¹0 Ma,indicating a cooler and drier climate.This phenomenon is confirmed by other evidence from Asia,e.g.pollen record in Tianshui Basin,carbonate?¹⁸O in Linxia Basin and ⁴⁰Ar/³⁹Ar ages in Tuha Basin.The increase of conifers at 10-8 Ma indicates the existence of high altitude mountain habitats in the periphery of the Qaidam Basin,most possibly the uplift of Qilian Mountains.The sudden increase of conifers and herbs resulted from a combination of long-term tectonic uplift forcing and global climate changes:a continuously cooler and drier climate may have caused the substantial intensification of physical weathering and exhumation on the earth's surface,leading to decline of CO₂ concentration,and the further drop of temperature by positive feedback.The uplift of Qilian Mountains may block water vapor,change general circulation and/or redistribute land-sea area to lead to arid and cold environment.?5?Multi-index has demonstrated the Lulehe Fm.is very different from the overlying strata.Lulehe Fm.is characterized by coarse grain sizes,low total carbon?TC?content especially low TOC content,high redness a*value,low lightness L*value and low magnetic susceptibility.No hiatus was found with the overlying Xia Ganchaigou Fm.in the fieldwork.The magnetobiostratigraphy age is>25.5-23.5 Ma.It may be a response to global climate cooling and consistent with the time of the initial formation of monsoon climate since the late Oligocene-early Miocene in China.An increase in magnetic susceptibility and decrese in organic carbon flux at¹1.5 Ma may indicate the uplift of Qilian Mountains,coupling with the appearance of detritus from the Qilian Shan recorded in detrital-zircon U-Pb ages at 12-10 Myr.?6?The grain size of Honggou section is characterized by silt dominance.According to grain-size distribution and accumulation curves,clay-silt-sand triangular plots,C-M plots,Q1-Md-Q3 plots,Sahu‘s empirical judgment equation and morphology of quartz grains,it is typical eolian deposits.Three end members were obtained from grain size data with the end member modeling.EM1 component is clay and fine silt,representing longer distance transport.EM2 component is silt,originating from the short-term suspension population,which could be used to represent winter monsoon intensity.EM3 component is coarse silt and sand,representing storm or fluvial transport.The EM2 curve is consistent with the global?¹⁸O curve,indicating the long-term intensifying trend of winter monsoon since 15Ma.