Dongmei Qu

Isotopic Signal of Earlier Summer Monsoon Onset in the Bay of Bengal

AbstractThe onset of the Asian summer monsoon is noticeably controversial, spatially and temporally. The stable oxygen isotope δ18O in precipitation has long been used to trace water vapor source, particularly to capture the summer monsoon precipitation signal. The abrupt decrease of precipitation δ18O in the Asian summer monsoon region closely corresponds to the summer monsoon onset. Two stations have therefore been set up at Guangzhou and Lulang in the East Asian summer monsoon domain to clarify the summer monsoon onset dates. Event-based precipitation δ18O during 2007/08 is much lower at Lulang than at Guangzhou and is attributable mainly to the altitude effect offset by different isotopic compositions in marine moisture sources. The earlier appearance of low δ18Owt at Lulang than at Guangzhou confirms the earlier summer monsoon onset in the Bay of Bengal. Isotopically identified summer monsoon evolutions from precipitation δ18O at both stations are verifiable with NCEP–NCAR reanalysis data, indicating...

Influences of relative humidity and Indian monsoon precipitation on leaf water stable isotopes from the southeastern Tibetan Plateau

This project report results of the first leaf water δ18O and δD time series of the woody plant Quercus aquifolioides undertaken on the southeastern Tibetan Plateau (STP). The data show that δD of leaf water underwent less enrichment than δ18O relative to precipitation and soil water, and could “inherit” the δD value characteristics of precipitation far better than δ18O. The leaf water δD values were strongly modified by evaporative enrichment in response to lower and more drastically changing relative humidity observed outside of the rainy season. In comparison, leaf water δD values more closely tracked those observed in the monsoon precipitation (acted as source water) in the rainy season. Our findings suggest that leaf water δD values from the STP were sensitive to the Indian monsoon activities, and the effects of relative humidity and the Indian monsoon precipitation should be considered for stable isotopes in tree ring cellulose or other biomarkers studies on the STP.

The Indian monsoonal influence on altitude effect of δ 18 O in surface water on southeast Tibetan Plateau

The altitude effect of δ18O is essential for the study of the paleo-elevation reconstruction and possible to be solved through modern process studies. This study presents new δ18O results from southeast Tibetan Plateau along two transects, the Zayu transect and the Lhasa-Nyang transect, with δ18O data from June to September representative of monsoon period and δ18O data during the rest of the year of non-monsoon period. Altitude effect outweighs the longitude and latitude effects in determining regional δ18O variation spatially. Relevant δ18O data from previous studies in the nearby region have also been combined to comprehensively understand the influence of different moisture sources on δ18O from local scale to regional scale. The δ18O in surface water in the southeast Tibetan Plateau and its nearby regions influenced by the Indian summer monsoon shows that single dominant moisture source or simple moisture sources lead to smaller altitudinal lapse rate, whilst growing contributions from local convection to precipitation enlarge δ18O-altitude rate. It thereupon reveals the significance of the Indian summer monsoon to the altitude effect of δ18O in surface water, and the complicated effect of local convection or westerlies evolution to the variation of altitudinal lapse rate. Paleo-monsoon evolution therefore should be considered when altitude effect is applied to paleo-elevation reconstruction for the Tibetan Plateau.

Lake Water Isotope Variation Linked with the In-Lake Water Cycle of the Alpine Bangong Co, Arid Western Tibetan Plateau

ABSTRACT Water isotopes play an important role in the study of the alpine lake water budget and the hydrological cycle in the arid, far western Tibetan Plateau. These isotope records, derived from well-preserved sediments, are believed to reflect climatic and environmental changes. Using two years of δ18O and δD data from precipitation, river water, underground water, and lake water at the long alpine lake Bangong (LBG), together with local meteorological observations in the arid western Tibetan Plateau, this study reveals that the δ18O in the lake is over 10‰ more enriched than that in the local precipitation due to evaporation of the lake water. Spatial changes in both the lake water δ18O and d-excess (= δD - 8 * δ18O) are apparent, ranging from ∼-4.9‰ to +0.9‰ for δ18O, and -13.22‰ to -30.85‰ for d-excess, respectively, from east to west of the lakes. Simulation with the Craig-Gordon model shows that the isotopes in alpine inland lake water are controlled to a great extent by local relative humidity. Using a modified partly mixed isotope fractionation model, we rebuilt the spatial change of the evaporation/inflow (E/I) ratios from east to west of the lake. A quantitative estimation shows that the E/I ratio of the lake water increases from 0.73∼0.83 in the eastern part and 0.90∼0.93 in the western part of LBG. We also found that by assuming a full development of kinetic fractionation of the environmental vapor isotopes, our simulation result matched the observed spatial change for both δ18O and d-excess, confirming the strong inland evaporation enrichment in the northern part of the plateau. This research may increase our understanding of inland water movement in the alpine Tibetan lakes, and also will improve our understanding of the lake sediment isotope record.