祁连山东段北麓近10 ka来的气候变化初步研究

CLIMATIC CHANGES IN THE NORTH PIEDMONT OF EASTERN QILIAN MOUNTAINS SINCE 10 ka B.P.

磁化率、碳酸钙含量和有机碳含量可用作良好的气候指标，主要是反映了降水量的变化。祁连山东段北麓哈溪剖面研究表明，９５６０ａＢ．Ｐ．时本区已进入全新世。大暖期是对温度而言，与气候适宜期是不同的概念，二者在时间上也不一定是同时的。本区的气候适宜期为６．８～３．６ｋａＢ．Ｐ．，气候最宜期为５．０～３．６ｋａＢ．Ｐ．，与其它记录有不同之处。在７．８～７．２ｋａＢ．Ｐ．和１．５～０．８ｋａＢ．Ｐ．间出现了“冷湿”的气候组合，这与传统的气候组合不同，但后一阶段的“冷湿”是有大量证据的。９．０～８．７ｋａＢ．Ｐ．间是全新世最干旱的时段，堆积了砂黄土。

Holocene is one of the most important period in PAGES. Haxi section sites on the first terrace of Haxi river in the north piedmont of eastern Qilian Mountains, and it has a thickness of 324 cm for aeolian loess and palaeosol. Susceptibility, CaCO 3 content and Organic Carbon content are used as the good indexes of climatic changes(mainly the precipitation changes) in loess palaeosol research. Study on Haxi section indicates that in this area it entered Holocene before 9560 a B.P.. In the early Holocene ,climate changed frequently and had a obvious instability. The climate suitable stage and climate optimum took place within about 6.8￣3.6 ka B.P. and about 5.0￣3.6 ka B.P. respectively; scores of climatic indexes kept high during these two stages. There was a cold but still a little humid stage during 6.0￣5.0 ka B.P., while evidences from other region showed it was cold and dry in those area during this period. After 3.6 ka B.P., it became drier. In 1.5￣0.8 ka B.P., score of susceptibility was high than that of the climatic optimum, but the leaching depth of CaCO 3 was shallow than the latter. These conclusions are different from other climate records and traditional climate characteristics. In arid and semi arid region of northern China, the increase of summer monsoon will engenders more precipitation and an increase of temperature; but, the higher temperature may increase more evaporation and reduce humidity . So the Megathermal Maximum is different from the climatic optimum in conception ,the former referred to temperature and the latter to assembly of climate(mainly water and thermal).And those two maybe not occurred in the same time. Comparison with Dunde Ice-core record shows it was cold wet during 7.8￣7.2 ka B.P. and 1.5￣0.8 ka B.P., especially the latter has plenty of proofs. During the Megathermal Maximum(7.2￣6.0 ka B.P.), the environment was not at the best (optimum) in the whole Holocene because of intense evaporation. In the climatic transition processes(from cold to warm or from warm to cold), it was usually dry. It was the most dry stage during 9.0￣8.7 ka B.P. when the climate changed abruptly and drastically(from the coldest to the warmest within 200 years)and deposited silt loess. The precipitation reached its maximum during 5.0￣3.6 ka B.P., and, an ancient civilization was flourishing in our research region. Many researches indicate that during 5.0￣3.6 ka B.P. it was warm and wet in other regions, such as in eastern and northern China. In arid and semi arid regions, water is the most important environment factor, so, warm wet climate assembly may be the finest of climate in these areas.