An 8.1Ma calcite record of Asian summer monsoon evolution on the Chinese central Loess Plateau

Carbonates in loess-red clay sequences consist mainly of calcite and dolomite. The EDTA analysis of carbonates in different size fractions and magnetic susceptibility reveal that calcite is a sensitive index of summer monsoon. The chemical analysis of carbonates and calcite from an 8.1 Ma loess-red clay sequence at Chaona on the Chinese central Loess Plateau shows that the evolution of the Asian summer monsoon experienced four stages, namely 8.1―5.5 Ma, 5.5―2.8 Ma, 2.8―1.5 Ma and 1.5―0 Ma, with increasing intensification and fluctuation, suggesting a possible combining impacts of uplift of the Tibetan Plateau and global changes on the Asian summer monsoon.     

Climate control on summer precipitation in the Source Region of Three Rivers,China: East or South Asian summer monsoon?

ABSTRACT

The Source Region of Three Rivers (SRTR) has experienced wetter summer seasons than before in recent decades due to climate change. As the most important source of surface water, precipitation plays a key role in supplying the three largest rivers. This study investigates the impacts of the East Asian summer monsoon (EASM) and the South Asian summer monsoon (SASM) on precipitation in the SRTR. Using wavelet analysis tools, we found that: (i) summer precipitation in the SRTR showed notably different responses to the monsoon variability among the 14 stations studied; (ii) the influence of the EASM and SASM on summer precipitation was stronger in the southern and eastern SRTR; but (iii) this influence quickly dampened from southeast to northwest and became almost indiscernible in the northwestern SRTR. This research may help to increase the accuracy of long-term monsoon-rainfall prediction and improve water resource management in the SRTR.     

Possible mechanism of the effect of convection over Asian-Australian “land bridge” on the East Asian summer monsoon onset

The Asian-Australian “land bridge” is an area with the most vigorous convection in Asian monsoon region in boreal spring, where the onset and march of convection are well associated with the onset of East Asian summer monsoon. The convection occurs over Indo-China Peninsula as early as mid-April, which exerts critical impact on the evolution of monsoon circulation. Before mid-April there are primarily sensible heatings to the atmosphere over Indo-China Peninsula and Indian Peninsula, so the apparent heating ratios over them decrease with height. However, after mid-April it changes into latent heating over Indo-China Peninsula due to the onset of convection, and the apparent heating ratio increases with height in mid- and lower troposphere. The vertical distribution of heating ratio and its differences between Indo-China Peninsula and Indian Peninsula are the key factors leading to the splitting of boreal subtropical high belt over the Bay of Bengal. Such mechanism is strongly supported by the fact that the evolution of the vertical heating ratio gradient above Indo-China Peninsula leads that of 850 hPa vorticity over the Bay of Bengal. Convections over Indo-China Peninsula and its surrounding areas further increase after the splitting. Since then, there is a positive feedback lying among the convective heating, the eastward retreat of the subtropical high and the march of monsoon, which is a possible mechanism of the advance of summer monsoon and convection from Indo-China Peninsula to South China Sea.     

Applications of interannual-resolution stable isotope records of speleothem: Climatic changes in Beijing and Tianjin, China during the past 500 years—the δ 18O record

δ¹⁸O of a stalagmite collected from Shihua Cave, 50 km southwest of Beijing is analyzed. The uppermost 2 cm was sampled at about 3-year intervals by a computer-controlled microsampling device. A total of 133 samples were analyzed, covering the last 480 years. A comparison of the δ¹⁸O record with the instrumentally recorded precipitation in Beijing and Tianjin back to 1840 AD shows that high precipitation correlates with negative δ¹⁸O peaks. The long-term δ¹⁸O trend records temperature changes. Between 1620 and 1900 AD, the temperature was cooler than the average value for the 480-year record, corresponding to the Little Ice Age. Temperatures warmer than the average prevailed during 1520–1620 and 1900—present. Superimposed on the long-term trend are about 14 δ¹⁸O cycles of 30–40-year periodicity, with wet periods centered around 1985, 1955, 1910, 1880, 1840, 1800, 1760, 1730, 1690, 1660, 1630, 1600, 1560 and 1530 AD. Project supported by the National Natural Science Foundation of China (Grant No. 9615875).     

Strengthened East Asian summer monsoons during a period of high-latitude warmth? Isotopic evidence from Mio-Pliocene fossil mammals and soil carbonates from northern China

The East Asian monsoons have fluctuated in concert with high-latitude warmth during the past several hundred thousand years, with humid summer monsoon-dominant climates characterizing warm intervals, including interglacials and interstadials, and arid winter monsoon-dominant climates characterizing cool intervals, including glacials and stadials. Of the states comprising the mid-Pleistocene to recent climatic regime, interglacials are most similar in terms of high latitude ice volumes and temperatures to those extant during the late Miocene and early Pliocene. Thus, an important question is whether Mio-Pliocene climates in northern China were analogous to a hypothetical ‘prolonged interglacial state,’ with increased summer monsoon precipitation and expansion of forest and steppe environments at the expense of desert environments.We utilize new and previously published carbon isotopic data from fossil teeth and soil carbonates to place constraints on paleovegetation distributions and to help infer the behavior of the monsoon system between ～ 7 and 4 Ma. We find that plants using the C₄ photosynthetic pathway—which today are largely grasses found in regions with warm season precipitation—were present in northern China by late Miocene time, demonstrating that the C₄ expansion in China was not significantly delayed compared to the global C₄ event. During the late Miocene–early Pliocene interval, soil carbonate and tooth enamel δ¹³C data indicate: 1) that nearly pure C₃-plant ecosystems existed in the southern Chinese Loess Plateau (CLP), and therefore ecosystems there were dominated by woody dicot, herbaceous dicot, or cool-season grass vegetation (or a combination of these), and 2) that the CLP was characterized by a pattern of northward-increasing C₄ vegetation and aridity. Utilizing a broadened conceptual model for interpreting δ¹³C data, and citing independent faunal, floral, and lithostratgraphic data, we suggest that these patterns reflect northward expansion of forest and steppe ecosystems and relatively humid monsoon climates during the late Miocene and early Pliocene. An important implication of this interpretation is that the forcing mechanism illuminated by the temporal correlation during the Pleistocene between warm high latitudes and strong East Asian summer monsoons is a robust feature of the Eurasian tectonic–climatic system that predates the Plio-Pleistocene climatic reorganization.