Organic Geochemical Records Of Mid-late Cenozoic Paleoelevation And Paleoenvironments In The Southern Tibetan Plateau

The uplift of the Tibetan Plateau greatly impact on the surrounding region and even the global climatic conditions,especially laying the foundation of the Asian climate pattern.There are still great debates on the history and mechanism of plateau uplift especially paleoheight.The onset,and development of the Asian monsoon and their mechanism are very important scientific issues.Therefore,The study of paleoelevation and paleoenvironmental evolution of the Tibetan Plateau is helpful to understand the interconnection between the uplift process of the plateau and the monsoon climate in Asia.In this paper,we analyzed the carbon and hydrogen isotope compositions of leaf waxes and branched glycerol dialkyl glycerol tetraether lipids(br GDGTs)in three regions including the southeastern margin of the Tibetan Plateau,Nima basin in the southern plateau,Site U1451 in middle of the bengal fan where the environment of southern plateau was recorded.We did the following researches by the above three organic proxies: firstly,we have studied leaf wax ?D,?13C and br GDGTs derived MBT/CBT temperature changes with elevation in modern soils along the altitudinal slope.Secondly,the paleoheights of the Nima basin were comprehensively reconstructed based on the organic indexes.We reestablished the environmental evolution of southern plateau,and explore the relationship between the southern Plateau uplift and the South Asian monsoon.Here is achievements we have.The measured lapse rate of mean annual air temperature(MAAT)in the region was-7.2 °C/km,whereas that estimated fromthe MBT-or MBT'–CBT index was lower,-5.0 °C/km and-3.2 °C/km respectively.We calculated the mean warm season temperature that only includes months with temperatures >0 °C at the 8weather stations,and then determined a mean warm season temperature lapse rate of-3.1 °C/km.Clearly,this value is consistent with the apparently low lapse rate from the MBT'–CBT derived temperature values,implying the warm season bias of MBT'–CBT temperature proxy.Due to the mixture of C3 and C4 plants in the plateau,our results showed the relatively high scatter of the linear relation between ?D and altitude and no trend between ?13C and altitude This suggests that care should be taken in interpreting br GDGT derived temperature values with seasonal bias in environments and reconstructing paleoelevation with large seasonal contrasts.?D and ?13C of n-alkane in reconstructing paleoelevation should avoid the C4 plant.Lipids analysis on sedimentary rocks in the Nima basin yielded-222 ± 12‰ and-30.0 ± 0.7‰ for ?D and ?13C of C29 n-alkane,respectively,and b GDGT-derived mean annual air temperature(MAAT)of 20.7 ± 2.9 °C for the paleo basin.Using the meteoritic water ?18O of tropical ocean,paleovegetation ?13C of lowland Siwalik and as the low elevation references,values of ?D and ?13C of C29 n-alkane in this study could suggest paleoelevations of 4546 ± 340 m and 2800 ± 629 m,respectively.Whereas by using the tropical sea surface temperature as a reference and a regional terrestrial temperature lapse rate of 5.6 ± 1.0 °C/km,the br GDGT-derived MAAT may suggest a paleoelevation of 1340 ± 550 m.We believed that the three estimates reflect a high-relief topography in the study basin during the mid-Cenozoic,representing elevations of orographic barrier of southerly oceanic moisture,basin catchment and basin floor,respectively.This scenario implies a low elevation of the basin relative to Lhasa Terrane located in the south during the mid-Cenozoic,hence challenging the classical crustal thickening models assuming a simple crustal thickening-uplift relationship.We reconstructed the evolution of paleo-precipitation,paleovegetation and terrestrial temperature in south Asian and the sea surface temperature in Bengal ocean by hydrogen and carbon isotopic compositions,of plant wax n-alkanes and branched-chain and isoprenoid glycerol dialkyl glycerol tetraether lipids(?D,?13C,br GDGTs and iso GDGTs)respectively.7~5Ma,?D values markedly increased by ~30‰,and ?13C values concurrently rised by ~5‰,which suggested that the rapid decrease in the precipitation drove the late Miocene C4 plant expansion.After 15.5Ma,MAT continually decreased by ~6 °C,and SST was increasing by ~3 °C,which showed the asynchronous variability between both.7~3.5Ma,the difference both SST and MAT rapidly increased from ca.4 °C to ca.8 °C,likely implying the rapid uplift in the southern plateau.There were the consistent changes among the precipitation,vegetation and the difference both SST and MAT in south Asia during the late Miocene.Therefore,it suggested that the rapid uplift of the Tibetan Plateau in the late Miocene resulted in the change of the thermal pattern distribution of the sea and land,which controlled the decrease of summer monsoon precipitation in South Asia,resulting in the expansion of C4 plants.