末次盛冰期北美风成环境变化及其在全球气候突变事件中的意义

VARIATIONS IN THE EOLIAN ENVIRONMENT OF NORTH AMERICA DURING THE LAST GLACIAL MAXIMUM AND THEIR SIGNIFICANCE IN THE GLOBAL ABRUPT CLIMATIC EVENTS

本文用末次冰期北美大冰盖南缘的风尘堆积记录来重建末次盛冰期的亚极地环境变化。磁化率、有机碳含量和潜育化程度的资料表明 ,末次盛冰期的前期 (约 2 1 0 0 0～1 5 0 0 0aB .P .)植被覆盖较差 ,而后期 ( 1 5 0 0 0～ 1 0 0 0 0aB .P .)植被覆盖较好 ,1 5 0 0 0aB .P .环境发生了急剧的变化 ,磁化率曲线显示了大约 1 0 0 0a的氧化 -还原周期。这种大约 1 0 0 0a的环境变化周期很可能由低纬地区的海洋和大气的扰动所驱动 。

Based on the negative correlation of the susceptibility with organic matter content and the degree of gleying, this study suggests that the reducing conditions contributed to the alteration of magnetic minerals from strong forms of oxidized iron to weak forms. Therefore, the magnetic susceptibility is basically an indicator of redox cycles. Well vegetated conditions generally dominated the later part of the last glacial maximum (15?000～10?000aB.P.)while poorly vegetated conditions during the early part of the last glacial maximum (21?000～ 15?000 aB.P.). The dark blue zone dated at 14?500 ～14?800aB.P. along the southern margin of the North American icesheet chronologically corresponds to a pollen and insect indicated climate amelioration in the Midwest of United States and to a period of C 4 plant domination in the central Great Plains. The transition may also be causally related to the drastic changes in ventilation of the deep North Atlantic water and to an extreme aridification in Africa around 15?000aB.P. The magnetic susceptibility signatures indicate about 1?000 year long cycles of eolian environmental changes during the last glacial maximum (21?000～ 10?000aB.P. ). The cycles seem to be well corroborated by about 1?000 year long cycles recorded in the South China Sea, Santa Barbara Basin, and Greenland. The initial perturbation of these～1?000 year long cycles might have started in the Tropical Pacific and then promptly transferred to the North Atlantic via the North American Icesheet ( i. e ., Laurentide Icesheet). That is, long term ENSO variations in the Tropical Pacific probably controlled North American Icesheet dynamics by influencing precipitation and perhaps temperature as well. Icesheet dynamics might, in turn, have influenced the North Atlantic sea surface temperature via iceberg discharges. Consequently, the North Atlantic might have controlled the high latitudes and Tropical Pacific and other tropical waters the low latitudes. The middle latitude lands might have been influenced by the Tropical Pacific related and/or the North Atlantic related events depending on their geographic and climatic positions. $$$$