Age model and grain size distribution of the fine fraction of ODP Site 114-704

The physical properties of sediments beneath an upwelling area in the southern part of the Atlantic Ocean (ODP Hole 704A) were investigated. Highly significant correlations characterize the relationship of carbonate content to bulk density (R = 0.85), carbonate content to porosity (R = 0.84), and carbonate content to impedance (R = 0.84). No relationship exists between carbonate content and compressional-wave velocity (R = 0.24), indicating that amplitude variations in impedance are primarily controlled by variations in bulk density, which, in turn, are controlled by climatically driven biogenic opal and carbonate deposition. In general, maxima in impedance correspond to maxima in carbonate content (minima in opal content). The impedance record exhibits its most drastic change at about 2.4 Ma, marking dramatic increases in the average content of biogenic opal and the beginning of large-amplitude fluctuations. Between 0.7 and 0.4 Ma carbonate content, bulk density, and grain density decrease while opal content drastically increases. Similar changes have been observed in sediments beneath an upwelling cell off northwest Africa, indicating an oceanwide enhancement in upwelling or in the calcite corrosiveness of bottom water that appears to be isochronous.本研究针对大西洋南部上升流区下方的沉积物（大洋钻探计划（Ocean Drilling Program，ODP）704A孔）的物理性质展开了系统调查。碳酸盐含量与容重（bulk density，R=0.85）、碳酸盐含量与孔隙度（porosity，R=0.84）以及碳酸盐含量与阻抗（impedance，R=0.84）之间存在高度显著的相关性。碳酸盐含量与纵波速度（compressional-wave velocity）之间则无明显关联（R=0.24），这表明阻抗的振幅变化主要受控于容重变化，而容重变化又由气候驱动的生物成因蛋白石（biogenic opal）与碳酸盐沉积所调控。总体而言，阻抗峰值对应碳酸盐含量的峰值（即蛋白石含量的谷值）。阻抗记录在约2.4 Ma处发生最为剧烈的突变，标志着生物成因蛋白石平均含量的显著提升，以及大振幅波动序列的开端。在0.7 Ma至0.4 Ma期间，碳酸盐含量、容重与颗粒密度（grain density）均出现下降，而蛋白石含量则大幅升高。在非洲西北部近海上升流区下方的沉积物中也观测到了类似的变化特征，这表明全球大洋范围内的上升流增强或底层水方解石腐蚀性提升现象似乎是等时的。