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Stalagmite δ~(18)O Record Of Climate Change On Eastern Qinghai-Tibetan Plateau, China

Posted on:2008-09-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:P Z ZhangFull Text:PDF
GTID:1100360215457774Subject:Physical geography
Abstract/Summary:PDF Full Text Request
Too short Instrumental record (~150 year) makes us to produce many uncertainties in understanding past climate, studying current climate and forecasting future climate. The objective of my dissertation research is to explore precise-dating and high-resolution stable isotopic record from Wanxiang Cave (33°19'47''N, 105°00'19''E, 1200 m a.s.l.) on eastern Qinghai-Tibetan Plateau, China during recent 2000 years and Marine Oxygen Stage 5 (MIS 5).Speleothems are well suited for high-resolution paleoclimate reconstructions as they are generally very pure, well-preserved, recorded paleoenvironmental proxy data in highly resolvable growth band, and can be precisely dated with the 230Th method. The climate of the eastern Qinghai-Tibetan Plateau, linking Qinghai-Tibetan Plateau to Loess Plateau, is significant because it is under the influence of the East Asian monsoon system, an important component of the global climate system. The climatic and hydrologic patterns in this region are very complex, and thus difficult to model, hence more accurate and detailed proxy climate data are necessary.Previous records have exhibited a general pattern of increased Asian monsoon intensity during interglacial or interstadial periods, and decreased intensity during glacial or stadial periods. Moreover, this pattern still remains many uncertainties on shorter time-scale. The goal of my dissertation was to discuss the real pattern, amplitude and cause of Asian monsoon variability during recent 1810 year, and relationship between Asian monsoon variability, Greenland ice-coreδ18O record from higher altitude of Northern Hemisphere and the speleothemδ18O record from Southern Hemisphere during MIS 5, on the base of detailed study of Modern oxygen isotopic systematics between water and carbonate in Wanxiang Cave, Huanglong Cave and Huangcao Cave from the eastern Qinghai-Tibetan plateau. The following conclusions will be withdrawn:1. To assess the potential for obtaining of high-resolution paleoclimate record from the stable isotopic variation (δ18O) in fossil speleothem calcite, the result of a through study of the active water-carbonate system of Wanxiang Cave, Huanglong Cave and Huangcao Cave located on the eastern Qinghai-Tibetan Plateau. Theδ18O andδD of all cave waters and local precipitation samples plot directly on the local meteoric water line, suggesting that there is a close relationship between theδ18O of the cave water and theδ18O of modern precipitation. Using the measuredδ18O values of cave drip water and modern soda straw calcite and the carbonate paleotemperature equation, the calculated temperature of 12.0℃in the deepest part of the cave is between measured temperature (10.99℃) and mean annual temperature (14.4℃); and the calculated temperature of 6.9℃in the Huanglong Cave is close to measured mean cave temperature (6.2℃) during dominant monsoon precipitation between May and September. This suggests that modern speleothems are forming under isotopic equilibrium and quis-equilibrium and that their composition reflects changes in temperature at the surface and changes in drip water composition. Fossil speleothems form two caves are, however, well suited forδ18O based paleoclimate reconstructions, as theδ18O of speleothem calcite is inversely related to summer monsoon intensity at this region.2. The history of Asian summer monsoon variation is presented by high-precise absolute 230Th dating oxygen isotopic composition of WX42B collected in Wanxiang Cave, Gansu with mean resolution of 2.5 years.A comparison of the stalagmiteδ18O record with instrumentally meteorological data during AD 1951-1984 indicates that shifts of theδ18O are largely controlled by the amount effect of meteoric precipitation conveyed through Southwest monsoon (the Indian monsoon) and less affected by temperature. Therefore, the variations ofδ18O record reflect the changes in monsoon precipitation on at least five time scales under the influence of Southwest monsoon. Like many other speleothemδ18O records in Asian monsoon regions, theδ18O record of the stalagmite from Wanxiang Cave also reveals a gradually enriched trend during the past more than 30 years, i.e., relatively enriched in 18O. This trend may indicate the decline of Asian monsoon intensity which is consistent with the decrease of monsoon indices. The weakening of modern Asian monsoon well matched with the temperature changes in stratosphere, which may illustrate that the weakening of the monsoon is mainly resulted from the lowering of solar radiation.The long period of WX42Bδ18O time-series can also present four distinct sections in more negativeδ18O (AD 191-825, AD 950-1045, AD 1090-1315, and AD 1875-1984), and three distinct sections in more positiveδ18O and three distinct sections in more positiveδ18O (AD 825-950, AD 1045-1090, and AD 1315-1875), representing increased period of Asian monsoon and decreased period of Asian monsoon, respectively. So-called the Medieval Warm Period (MWP) with increased intensity of summer monsoon is between AD 950-1315 with 465 years of duration and dramatic weakening summer monsoon occurs between AD 1045-1090; and following the Little Ice Age (LIA) occurs AD 1315-1875, characterized by more positiveδ18O values, generally exhibiting decreased intensity of Asian monsoon. Every period of increased sommer monsoon is almost characterized by abrupt beginning of more negativeδ18O values and end of becoming more positiveδ18O values, indicating the features of abrupt beginning and steplike weakening of Asian monsoon. Some similarities are presented through comparison between theδ18O record and reconstructed mean North Hemispheric temperature. Likewise, stalagmiteδ18O record match well with solar output; abrupt more negativeδ18O (abrupt increased summer monsoon) at AD 530, AD 950 and AD 1090 during Dark Age Cold Period (DACP) and MWP, and at AD 1615 during LIA are corresponding to minimum 14C production and/or maximum changes in solar modulation function, suggesting Asian monsoon variability during recent 1810 year is strongly controlled by change of solar activity not only on century-scale but also 5-10 year scale. At same time, the cyclical variability of Asian summer monsoon may synchronously tele-connect to change in winter Northern Atlantic Oscillation (NAO) phase and deep-water temperature of North Atlantic, closely link to renew of water mass in North of northern sea, i. e., abrupt increase of precipitation amount-abrupt strengthening of summer monsoon and abrupt decrease of precipitation amount-abrupt weakening of summer monsoon are corresponding to rising temperature and dropping of ocean deep water, respectively. This type of the tie linkage between Northern Atlantic climate in higher latitude and Asian summer monsoon in middle-low by westerly wind zone consists of the coupled climate system between Sun-ocean-atmosphere in Northern Hemisphere.3. We also report high resolutionδ18O records of MIS5 (from 79kaB.P. to 118kaB.P.) from two stalagmites (WXSM51 and WXSM52) collected form the Wanxiang Cave, Gansu Province (33°19'N, 105°00'E). The results indicate thatδ18O of stalagmite at this site is inversely related to the East Asian summer monsoon intensity. Theδ18O record of the Wanxiang Cave closely resembles the D3 record from the Dongge Cave, and also agrees well with the NGRIP record and the 65°N summer isolation curve. These indicate thatδ18O of the Wanxiang Cave is a valid and robust proxy for paleomonsoon intensity analysis. These also suggest that the East Asian monsoon varies in step with global climate change and is largely controlled by solar variations in the Northern Hemisphere.The Wanxiang record presents similarity to the Mediterranean speleothem records, and presents an inverse pattern compared with the Brazilian stalagmite BT2 record on the millennial timescales. These indicate that there are climate links among the East Asian monsoon, the Mediterranean and the Brazil summer monsoon regions, and further prove that the north-south climate seesaw did exist.It should be pointed out that the pattern of the transition from MIS5b to MIS 5a recorded in the Wanxiang records differs from that in the Shennongjia stalagmite records. This difference, which does not seem to be caused by sampling resolution difference, may indicate thatδ18O of the precipitation in the Wanxiang Cave region is more sensitive to changes in monsoon intensity due to its close location to the northern limit of the monsoon region.The stalagmite oxygen isotopic record of Wanxiang Cave in this research provides the history of Asian summer monsoon variation since 1810 year and during MIS 5. In particular, it could ultimately leads to improve the status without precisely absolute dating and available proxy in Asian summer monsoon region since 1810 years for long time. It plays some active role for accurately understanding the history of Asian summer monsoon on millennium timescale by stalagmite's oxygen isotopic composition. The monsoon record is strong related to solar radiation and NAO, indicating the monsoon climate in low latitude synchronously presents tele-connection with Northern Atlantic climate in higher latitude. Our results may, therefore, be useful for better understanding of the forcing mechanisms behind the Asian monsoon and its role in the global climate system, more forecasting of future climate change.
Keywords/Search Tags:Qinghai-Tibetan
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