Late Quaternary Climate History on the Northeast Tibetan Plateau: Multiproxy Investigation of Lake Qinghai Sediments, China
2011-09
View/Download File
Persistent link to this item
Statistics
View StatisticsJournal Title
Journal ISSN
Volume Title
Title
Late Quaternary Climate History on the Northeast Tibetan Plateau: Multiproxy Investigation of Lake Qinghai Sediments, China
Authors
Published Date
2011-09
Publisher
Type
Thesis or Dissertation
Abstract
The objective of this dissertation is to reconstruct the history of the Asian monsoon and to examine the controls of the climate system on the Tibetan Plateau, using lake sediment cores from Lake Qinghai, China. Lake Qinghai is the largest inland water body in China, situated on the northeastern margin of the Tibetan Plateau, where the climate conditions are mainly controlled by the interaction of the East Asian monsoon and the Westerlies. It lies near the limit of penetration of the Asian summer monsoon, and is thus sensitive to climate changes. Yet the climate history in this region is not fully understood. This dissertation contributes to our understanding of a detailed Holocene and late glacial climate history in western China. A set of Lake Qinghai sediment cores, including a 18.6-m-long drill core (LQDP05-1F), a 3.5-m-long Uwitec sediment core (QH07-1A), and a 0.85-m-long mini-Mackereth core (QH07-1B-1MM), provide a record of climate that extends further back in time than that from any other records for Lake Qinghai. Results from multiple proxies derived from the composite 2007 core (QH07) are internally consistent and reveal a distinct Holocene and Late Pleistocene climate record. Carbonate content and total organic carbon in sediments are interpreted as proxies for the strength of the Asian summer monsoon. During the glacial period (~14,600 to ~20,000 yrs), the summer monsoon intensity remained low and relatively constant, suggesting cool, dry, and relatively stable climatic conditions. The Holocene (~11,500 yrs to present) was a time of enhanced summer monsoon strength and greater variability, indicating relatively wetter but more unstable climatic conditions than those of the Late Pleistocene. The warmest, wettest part of the Holocene occurred from ~9,000 to ~11,500 yrs. The transition between the Holocene and the Late Pleistocene, about 11,500 years ago, was abrupt. A cool Younger Dryas appears to be recorded in the record, but its onset is not as distinct as it is in cave records from Dongge and Sanbao. Evidence of a warm interval correlative with the Bølling–Allerød oscillation is weak in the QH07 record. We propose that changes in the contrast of summer insolation between the continent and the ocean are the primary control on the Asian monsoon system over the glacial/interglacial time scales. Secondary influences may include ice sheet size (albedo) and sea level (distance from moisture source). A climate threshold for arrival of monsoonal rainfall is suggested at the northeastern Tibetan Plateau. Magnetic properties and geochemistry of sediments were determined for an 18.6 m uchannel sample of core LQDP05-1F and several selected discrete samples. These results provide clues to changes in magnetic mineral concentration, grain-size distribution, mineralogy, and geochemical composition, as well as having implications for paleoclimatology. The relative abundances of iron and concentration-dependent magnetic parameters reflect higher concentrations of magnetic minerals during glacial times than the Holocene. Hysteresis measurements of the discrete samples show larger proportions of single domain (SD) minerals relative to multiple domain (MD) particles in Late Pleistocene sediment compared to Holocene sediment, suggesting that the glacial period was dominated by deposition of fine-grained aeolian materials, whereas the Holocene was characterized by increased riverine transport of coarse materials to the lake. Furthermore, greater variability of magnetic parameters and geochemical composition during the Holocene suggests complex and multiple sediment sources. Magnetite has been identified as the primary ferrimagnetic mineral throughout the core, suggesting relatively constant mineralogy. Relatively low magnetite concentration during the Holocene is mainly due to dilution by increased authigenic carbonate that is strongly associated with riverine Ca delivery. The presence of monoclinic pyrrhotite implies reducing depositional environment associated with remineralization of organic matter in the lake. Several lines of evidence suggest the occurrence of greigite, which may indicate relatively dry climate conditions during the glacial period. Results from the elemental composition and magnetic properties of the Lake Qinghai sediments are consistent with records derived from lithological and sedimentological proxies. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Fluorescence (XRF) scanning techniques were evaluated, using sediments from Lake Qinghai and Lake Malawi (Africa). The results show statistically significant correlations between conventionally measured concentrations of carbonate (%CaCO3), total organic carbon (%TOC), and biogenic silica (%BSi), and absorbance in the corresponding FTIR spectral regions, as well as between conventional measurements and XRF elemental ratios including calcium: titanium (Ca/Ti), incoherent: coherent X-ray scatter intensities (Inc/Coh), and silicon: titanium (Si/Ti). Both FTIR and XRF techniques exhibit great potential to quantitatively assess concentrations of inorganic and organic components of lacustrine sediments. These results provide evidence that climate on the Tibetan Plateau has varied considerably, suggesting a relatively stable, cold, and dry Late Pleistocene along with a weak Asian summer monsoon, versus a relatively unstable, warm, and wet Holocene with a relatively strong summer monsoon. The results also highlight that the Asian monsoon system is driven by changes in the contrast of summer insolation between the continent and the ocean over the glacial/interglacial time scales.
Description
A dissertation submitted to the faculty of the Graduate School of the University of Minnesota by Xiuju Liu in partial fulfillment of the requirements for the degree of Doctor of Philosophy, September 2011. Advisors: Dr. Steven M. Colman, Dr. Erik T. Brown.
Related to
Replaces
License
Series/Report Number
Funding information
Isbn identifier
Doi identifier
Previously Published Citation
Suggested citation
Content distributed via the University Digital Conservancy may be subject to additional license and use restrictions applied by the depositor. By using these files, users agree to the Terms of Use. Materials in the UDC may contain content that is disturbing and/or harmful. For more information, please see our statement on harmful content in digital repositories.