东北太平洋胡安·德富卡隆起西翼晚第四纪以来的碳酸盐记录

LATE QUATERNARY CARBONATE RECORDS IN THE WEST FLANK OF JUAN DE FUCA RIDGE,NORTHEASTERN PACIFIC OCEAN

通过对东北太平洋胡安·德富卡隆起西翼沉积柱样中的碳酸盐含量和浮游有孔虫氧同位素测试以及浮游有孔虫丰度、溶解指数和粗组分的统计,发现在晚第四纪约65 ka以来碳酸盐含量变化幅度大(0.4%～77.2%),相差非常悬殊,但其变化未呈现出冰期时溶解作用减弱、间冰期时溶解作用强烈这样明显的旋回性.粗组分分析结果显示,地层中有明显的浊流沉积发育.另外,碳酸盐含量和浮游有孔虫丰度分析结果揭露了研究区的CCD在3 500m左右.研究认为,在约65 ka来水深浅于3 500m的区域碳酸盐含量的无规律性变化应与浊流沉积影响有直接的关系,而水深深于3 500m的区域碳酸盐含量主要受控于深海碳酸盐的溶解作用.

Based on the determination of the carbonate contents and the oxygen isotopic analysis of planktonic foraminifera and also from planktonic foraminiferal abundance, dissolution index and the contents of coarse fraction (including colorful volcanic and terrigenous sands) in the sediment cores from the west flank of Juan de Fuca Ridge, the northeastern Pacific Ocean, it has been found that the carbonate contents in the studied area change greatly between 0.4%~77.2%, the same as that in the other areas of the Pacific. The results of the oxygen isotope analysis indicate that the sediments belong to a period 65 000 aBP. The average value of carbonate contents is 49.8% in these cores during oxygen isotope stage 4, 57.9% during stage 3, 53.9% during stage 2, and 28.3% during stage 1. The variations in carbonate contents do not clearly show the pattern that the dissolution decreased in glacial periods while increased in interglacial periods, which has already been revealed in other areas of the Pacific, but the low carbonate content during stage 1 was consistent to the strong carbonate dissolution at that time in the Pacific. The analysis of coarse fraction reveals that there were several turbidite layers in the sediment cores. Furthermore, the changes of carbonate contents and planktonic foraminiferal abundances show that the carbonate compensation depth is about 3 500 m in this area. Our observations indicate that during the last 65 000 years the anomalous changes of the carbonate contents in the cores with water depth < 3 500 m are directly related to the sedimentation of turbidite, while those in the cores with water depth > 3 500 m are mainly controlled by the deep-sea carbonate dissolution.