晚更新世末期黄、渤海陆架沙漠化环境的形成

未次盛冰期气候寒冷，温度降低，冰川发育，海面下降，黄、渤海陆架全部出露，东海陆架的大部分也裸露成陆，并成为亚洲大陆的一部分。降低了的气温使蒙古高压得到进一步增强，经常给亚洲大陆带来冷而干的气流，吹蚀亚洲内陆，也吹蚀出露了的陆架。根据多年来在陆架地区获得的浅地层剖面仪测量记录，陆架发生沙漠化的证据有：统一海相地层的解体、大面积的混杂堆积、漫长的风蚀基面、休止角型沉积结构以及埋藏沙丘群的发现等。

SHELF DESERTIZATION ENVIRONMENT IN THE BOHAI AND YELLOW SEAS DURING THE LAST GLACIAL MAXIMUM

During the last glacial maximum, the global climate was characterized by very low temperature. Enlargement of the continental glaciers brought about the lowering of East China Sea-level for about 130m. The entire Bobal Sea, Yellow Sea and most of the East China Sea were exposed and became a part of the Asian continent.The main characteristics of the Bohai and Yellow Seas'shelf environment during the last glacial maximum were manifested in the following aspects.(1) Strengthened Mongolia high pressureThe reduction of temperature in the last glacial maximum resulted in the Mongolia high pressure in the northern Asia strengthened, frequently occurred as larger anticyclone rotated with the clockwise direction, thus bringing the cold and dry air masses into the Asian continental area. The prevailing high pressure eroded both the inland of Asia and the exposed shelf regions. It is clear that the strengthened Mongolia high pressure, longer duration of winter monsoon and the enhanced wind power and speed were the major exegetic force on the shelf regions at that time (The highest speed wind at modern Bohai Sea strait is up to 34m/s).(2) Reduced air temperature During the LGM period, the global air temperature dropped about 8-13 ℃. With the temperature reduction, the rainfall pattern changed and the liquid rainfall was replaced by snowfall. The reduction of runoff to the oceans made the sea-level lowered.(3) Colder water temperatureDuring the LGM period, the temperature of the Pacific Ocean surface dropped remarkably. The drop of water temperature and evaporation made the NW Pacific Ocean not to be the source of typhoon. The disappearing of typhoon caused the eastern China, the Bohai Sea, Yellow Sea and East China Sea shelf lose a great deal of rainfall, and made the shelf environment developing towards arid.(4) Retreated rivers The colder ice age climate, the reduced rainfall, the longer freezing season, the southward migration of the southern limit of sporadic permafrost and the lower eroded basic level, all combined to make the rivers difficult to keep synchronous with the sea-level dropping.(5 ) Weakened KuroshioWith the water temperature dropped, sea-level lowered, shelf exposed and Oyashio (cold current) southward replacement, caused the Kuroshio current to migrate southward, weakened, and thus resulted in the shelf drier.The evidence for the shelf desertization is summarized as follows.(1) Disintegration of integrated Marine Stratum The Bohai Sea and Yellow Sea area had taken place many times of transgressions and regressions, formed sea-land strata over lapped alternately. In the ice ages regression occurred and land strata deposited, and in interglacial or interstadial periods marine strata formed.Because the water depth of Bobal Sea is shallow, it is very sensitive to sea-level changes. Any apparent fluctuation of the world sea-level would be reacted in the Bohai Sea area. After every time of regression, desertization on the exposed shelves is possible to occur, forming special mixed accumulation and destroying former marine strata.(2) Mixed depositsIn the last glacial maximum, under the stronger and prevailed winter paleomonsoon action, the former shelf consisted of sea-land strata alternately was destroyed and massive accumulation of mixed deposits occurred, indicating the features of storm deposits.(3) Endless erosion surfaceThe shelf that became a part of the Asian continent after regression and consisted of loose thick deposits was easily eroded by the stronger wind and the sediment separation occurred, forming endless erosional surface.(4) Typical "angle of repose" textureThe "angle of repose" texture is the typical feature of aeolian deposits, and its angle usually ranges from 28 to 32 degrees. This is another evidence for the storm deposit that often occurred on the exposed shelves.(5) Buried dune groupsThere are many active dune groups near the Changli of the modern Bohai Sea coastal zone. The exposed Bohai Sea shelf must have produced the larger and vast active dune group under the paleomonsoon action that ad