Font Size: a A A

The Biomarker Perspective On Neogene Environmental Evolution In The Tianshui Basin,NE Tibetan Plateau

Posted on:2013-10-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:T J PengFull Text:PDF
GTID:1220330467964668Subject:Physical geography
Abstract/Summary:PDF Full Text Request
The Tianshui Basin, a representative basin located at the conjunction of NE Tibetan Plateau and Chinese Loess Plateau and at the peak of "Monsoon Triangle", is very sensitive to environmental change. Recently, it has been received intensive attention because of its excellent Cenozoic deposition and abundant mammalian fossils. Till now, much program has been achieved not only on the uplift history of Tibetan Plateau, but also on the regional environmental changes as well as the interaction between tectonic and climate. Although precise magnetostratigraphy and detailed studies on sedimentology, isotope geochemistry, paleontology, and pollen have been performed on this set of Cenozoic sediments in the Tianshui Basin, few corresponding biomarker work has been done. Furthermore, the origin of these fine-grained Miocene sediments in Tianshui basin remains controversial. For example, previous studies proposed that these sediments are loess and the presence of late Oligocene/early Miocene loess pushes the timing of initiation of inland Asian desertification from8Ma back to22or25Ma. In contrast, many subsequent sedimentological studies proposed that the set of fine-grained Miocene sediments were deposited under the conditions of mudflat/distal fan, floodplain and shallow lake environment in the Tianshui Basin. Given the advantages of the biomarker method and controversial issues mentioned above, research from the biomarker perspective not only has the potential to resolve the origin of Neogene sediment in the Tianshui basin and to illustrate environmental change in the NE margin of Tibetan Plateau, but also enrich the currently scarce application of biomarker analysis in Tianshui areas.Here we investigate the detailed biomarker characteristics of four Neogene sections (Yanwan, QA-I, Yaodian and Lamashan) from a North-South transect of exposures within Tianshui Basin and compare these molecular distributions with those published Quaternary loess to help resolve the disputed origin. Furthermore, we reconstruct Neogene climatic history of Northeastern Tibetan Plateau based on our biomarker records of the Tianshui Basin and published data from its surrounding regions. Last but not least, we tentatively discuss the coupled mechanisms among the Tibet uplift, global cooling and the interior Asian aridification based on the integrated Neogene record. Detailedly, the main results and conlusions are listed as follows:(1) A series of biomarkers have been detected in the Tianshui samples, most of them are n-alkanes, n-alkan-2-one, branched alkanes, alkyl cyclopentanes, alkyl cyclohexanes, alkylbenzenes, fatty acids, wax esters, long-chain alkenones, mid-ketones, steranes, triterpenoids and polycyclic aromatic hydrocarbons.(2) All sediments contain n-alkanes ranging from n-C15to n-C37. Most of them display a bimodal pattern, whereas several samples show a tri-modal distribution. The Tianshui samples ubiquitously show a high abundance of medium-chain n-alkanes, which originates from submerged/floating aquatic macrophytes or palustrine plants that were grown in lacustrine-fluvial conditions. So our results confirm the basis hypothesis.(3) After comparing our n-alkane distribution from the Tianshui Basin with the published Quaternary Loess, we found that n-C23and n-C25alkanes, sourced from either aquatic macrophytes or palustrine plants, are ubiquitous in the Miocene sediments from Tianshui Basin but are largely absent in Quaternary loess. Considering the very slight biodegradation of Tianshui samples, we ascribe the obvious difference between Tianshui Neogene samples and Quateranry loess from Chinese Loess Plateau to the different origin and depositional environment. So this striking difference between n-alkane distributions in the Tianshui samples and the Quaternary loess casts doubt on an eolian origin for the Tianshui samples and challenges the hypothesis of late Oligocene/early Miocene onset of Asian interior desertification.(4) We have detected a series of long-chain alkenoes with carbon37,38and39in the lacustrine sediments of Tianshui Basin. Subsequently, the lak surface water temperature (LSWT) and salt degree of late Pliocene Lamashan paleolake (3.6-2.6Ma) has been reconstructed based on previous culture experiment and empirical equation. The temperature ranges from10.24~19.35℃and the salt degree ranges from19.1-21.8g/L, respectively. These premilary results demonstrate the potential advantage of long-chain alkenones on the temperature construction in the Tianshui basin and more work needs to be done in the furture.(5) The integrated results from the organic geochemistry, elment geochemistry, magnetic susceptibility, CaCO3and color proxies reveal that the Neogene climate of Tianshui areas could be devided into six stages:During the interval of22(or17)-14.5Ma, climate is warm and humid, which is consistent with the Middle Miocene Climate Optimum (MMCO); From14.5to12.5Ma, Tianshui basin was occupied by the cold-dry climate, suggesting a regional response to the Middle Miocene Climate Transition (MMCT). Then, climate turned into warm-humid conditions again during the interval of12.5-10Ma. From10to6Ma, climate changed into arid. From6to4Ma, Tianshui climate showed the characteristics of humid and warm. During the period of4-2.6Ma, climate was obviously dry.(6) In addition, we reconstruct an integrated Neogene climatic history of NE Tibetan Plateau based on our and published biomarker records. The results reveal that NE Tibetan Plateau did not have a permanent drying trend until~4Ma. Before4Ma, climate was generally humid although this pattern was temporarily interrupted by two drying intervals at~14.5-12.5and~10-6Ma. Interestingly, each of the two drying intervals and the permanent drying trend correspond to one phase of global cooling and/or uplift event of the Tibetan Plateau, highlighting the importance and complexity of tectonic-climate interactions.
Keywords/Search Tags:Tianshui Basin, Neogene, Biomarker, Environmetnal Change, AsianAridification history
PDF Full Text Request
Related items