40～30ka B.P.中国暖湿气候和海侵的特征与成因探讨

40～30kaB.P.相当于末次冰期(75～10kaB.P.)中的大间冰阶或海洋氧同位素第3阶段的晚期(MIS3a),已有测年记录的冰芯、湖泊沉积、孢粉与古生物、黄土与沙漠地区古土壤、石灰岩洞穴中石笋、古河道沉积、滨海地区海相沉积等7种记录指示我国各区域的湿润程度即降水量高于现代,青藏高原和西北地区尤为显著.西部和华南地区的温度明显高于现代,但较多的孢粉研究者认为东北、华北和长江流域的温度略低于现代.降水量的增加对内陆水系的合并和外流的黄河和长江流域等产生重大影响.渤海西侧、长江三角洲南北与珠江三角洲南部出现重大海侵,当时海平面高度仅低于现代海平面8～10m.当时暖湿气候与海侵的主要动力是岁差周期导致的中、低纬度太阳高辐射所提供的热力促使冰盖消融萎缩,海洋扩涨,青藏高原热低压增强,吸引季风降水,中、低纬度区海洋加热蒸发,增强夏季风与西风环流的水汽含量导致我国全境暖湿.高分辨率的西昆仑山古里雅冰芯记录和南京汤山洞穴石笋记录表明气候有许多百年级高频振荡波动,不是稳定暖湿,要作更深入研究才能说明其表现和影响.

Warm-humid climate and transgressions during 40～30 ka B.P. And their potential mechanisms

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.