Xiangzhong Li

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Hydrological changes of the past 1400 years recorded in δD of sedimentary n-alkanes from Poyang Lake, southeastern China

Hydrogen isotopic compositions (δD) of plant leaf wax have been increasingly used for paleohydrological reconstructions over various geological timescales. Here, we analyzed downcore variations of δD values of long-chain C31 and C33 n-alkanes from Poyang Lake in southeastern China to infer hydrological changes recorded in terrestrial higher plants during the past 1400 years. Variations in δD values of long-chain n-alkanes were generally consistent with fluctuations of published rainfall proxy (Dry–Wet Index) in southern regions of the middle and lower reaches of the Yangtze River Valley. We suggest that the δD record from Poyang Lake is likely influenced by the regional rainfall variations, which is interpreted as a result of the combined effect of ‘amount effect’ and evapotranspiration. More negative δD values during ca. AD 1200–1500 with a higher amount of rainfall suggested a wetter ‘Medieval Warm Period’ (MWP), and more positive δD values at ca. AD 1500–1900 with a lower amount of rainfall indicated a relatively dry ‘Little Ice Age’ (LIA). This pattern of hydrological changes generally corresponded to the intensity of the Asian summer monsoon (ASM) as recorded in oxygen isotopic compositions (δ18O) of stalagmite from the Wanxiang Cave in central China, providing further support that ASM has played an important role in controlling hydrological changes during the MWP and LIA in monsoon-dominated southeastern China.

Carbon isotope and environmental changes in lakes in arid Northwest China

The stable carbon isotope composition (δ13C) of lacustrine sediments, which can record changes in past environmental conditions such as the C4/C3 terrestrial vegetation composition, has been widely used for the reconstruction of terrestrial ecosystems and global climate changes. It has also been widely used in paleolimnological studies in arid northwestern China. In recent years, however, an increasing number of studies have enriched the environmental significance reflected by the stable carbon isotope geochemistry of lakes; therefore, the interpretation of lake δ13C variations in paleoenvironmental reconstructions should be carefully evaluated from various aspects. In this paper, previous studies from the past several decades on the lacustrine δ13C variations in arid northwestern China were reviewed, and the possible mechanisms of the δ13C variations were discussed. Our study will provide a reference for the application of carbon isotope geochemistry in paleoenvironmental reconstructions.