350ka以来北冰洋波弗特环流演变及其沉积响应

Variations of the Beaufort Gyre and depositional responses in the Arctic Ocean since the 350 ka

波弗特环流(Beaufort Gyre)是决定北冰洋海冰运移方向和滞留时间的主要因素。依据沉积物的颜色旋回、Mn元素含量和有孔虫丰度,本文建立了阿尔法洋脊(Alpha Ridge)B85-D孔轨道尺度上的年龄框架,并深入分析了该孔晚第四纪以来冰筏碎屑(IRD>154μm)含量、组分及其源区的变化特点。结果表明,350ka以来最显著的变化发生在末次冰期(MIS2～4期),期间沉积物中Ca元素和白云石含量都接近于零,大部分来自班克斯岛、维多利亚岛和麦肯齐地区的碳酸盐岩碎屑没有在阿尔法洋脊沉积;随着波弗特环流的消亡,搬运陆源碎屑的海冰很可能沿着北美海岸线直接进入了欧亚海盆。而在末次间冰期(MIS5期),波弗特环流却十分流畅,并将大量碳酸盐岩碎屑限制在美亚海盆内,导致阿尔法洋脊沉积物中Ca元素含量急剧升高,与之对应的白云石含量高达16.4%。通过类比可知,在MIS6期波弗特环流的状态可能与MIS2～4期相似,而在MIS8和MIS10这两大冰期,波弗特环流却类似于MIS5期。显然,波弗特环流的存在与否并不严格遵循"冰期-间冰期"旋回,这种变化很可能是风场强度和海冰浓度双重作用的结果。

The Beaufort Gyre was usually suggested as a main factor to control the residence time and moving direction of sea ice in the Arctic Ocean. Based on records of sediment color cycle, Mn content and foraminiferal abundance, we built up the orbital timescale for the core B85-D from the Alpha Ridge, and discussed variations of ice-rafted debris （IRD〉154μm） in content, composition and provenance since the Late Quaternary. The results suggest that the most distinct variations occurred during the last glacial （MIS2-4） as remarked by the near zero contents of calcium and dolomite, indicating that the carbonate debris from Banks Is. , Victoria Is. and District of Mackenzie did not deposit at the Alpha Ridge. The carbonate debris, transported by sea ice, was most likely heading to the Eurasian Basin along the off shore of the North America during the last glacial, when the Beaufort Gyre vanished. In contrast, the carbonate debris was mostly trapped in the Amerasian Basin during the last interglacial （MISS）, when the Beaufort Gyre became strong so as to deposit relatively high content of calcium and dolomite, which was up to 16.4%. By comparison, is it indicated that the state of the Beaufort Gyre during the MIS6 was closely analogous to that during the MIS2- 4, but its state during the glaials MISS and MIS10 similar to its state during the interglacial MISS. Therefore, variations in the Beaufort Gyre are not strictly characterized by the glacial-interglacial cycles, which we speculate, to be modulated by the sea ice coneentrationas well as the wind pressure.