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WARM-HUMID CLIMATE AND TRANSGRESSIONS DURING 40~30 ka B.P.AND THEIR POTENTIAL MECHANISMS

Shi Yafeng ①② Yu Ge② (①Institute of Cold and Arid Regions Environmental and Engineering Research, Chinese Academy of Sciences, Lanzhou 730000; ②Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008)  
The period of 40~30ka B.P. should be corresponding to the later phase of the mega-interstadial of the last glacial period (75~10ka B.P.) or to the later phase of Marine oxygen Isotope Stage 3 (i.e. MIS 3a). Previous palaeocilmate studies based on ice cores, lake and river sediments, pollen and macrofossils, paleosol from loess and desert sections, stalagmites from karst caves, and sea level changes indicated that precipitation and humidity during 40~30 ka B.P. were higher than those today, particularly in the Tibetan Plateau and northwestern areas of China. Also, temperature in that period was significant higher than the present's in West China and the South China, but numerous palynological studies suggested that temperature was slightly lower than that today from Northeast China to the Changjiang (Yangtze) River valley. The increased precipitation would have important effects on the convergence of inland water systems and on the catchment change of the Changjiang River and the Yellow River. Large transgressions occurred in the western coasts of the Bo Hai and the delta areas of the Changjiang River and the Pearl River, where the sea level at that time was 8~10m lower than the present sea level. A higher July-insolation in the mid and low latitudes than today occurred during the period 40~30ka B.P., which was mainly due to changes in the perihelion of the orbital parameters. This insolation anomaly was possibly a major dynamics that caused the warm-humid climate and the transgressions in China. The strong heat energy would shrink the icesheets in the high latitudes and hence rise the sea levels globally. The hot low pressure enhanced over the Tibetan Plateau would induce monsoon airflow from the ocean and increase precipitation. The increased heat-evaporation cycling in the low-mid latitude oceanic area would lead to enhancing of summer monsoon. The reduced icesheets in the high-latitude would decrease the high-latitude cooling and make the Westerly more active than today. These combined effects would increase precipitation and temperature over China. High-resolution records of Guliya Ice Core from the Tibetan Plateau and stalagmites from Hulu karst cave in Nanjing revealed many century-scale climate fluctuations, suggesting that the climate during 40~30 ka B.P. should not be a stable warm-humid period. A more realistic climate sequence and a better understanding of the mechanisms will rely still on more high-resolution studies in the future.
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