New appraisal of radium 226 as a large‐scale oceanic mixing tracer
New appraisal of radium 226 as a large‐scale oceanic mixing tracer
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DOI：
10.1029/94jc00089
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发表时间：
1994-05
影响因子：
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通讯作者：
T. Ku;S. Luo
中科院分区：
文献类型：
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作者：
T. Ku;S. Luo
对地球化学海洋剖面研究 (GEOSECS) 探险期间收集的印度洋东部和中部的镭 226 和 Ba 数据进行了审查，并用于描绘这两个物种的地球化学循环。数据显示，虽然 226Ra 和 Ba 的颗粒吸收发生在海洋表层，但它们的再生可能主要发生在沉积物-水界面附近或下方。因此，在深海的大部分区域（除了靠近海底的区域），226Ra 可以被视为化学保守的。利用 226Ra 的地球化学特性，我们能够计算印度洋东部和中部的垂直 (z) 和水平（经向、x）涡流扩散率（Kz 和 Kx）和平流速度（Vz 和 Vx）及其变化。这些混合参数的绝对大小根据 226Ra 的衰减率进行校准。该计算是通过对三个属性的质量连续方程进行数值求解来完成的：226Ra、总盐和海水质量。计算中使用的网格尺寸（其中平流传输被视为沿着等密度表面）的尺寸为 Δx ≈ 700 km 和 Δz ≈ 400 m。模型计算表明，在GEOSECS横断线沿线的东印度洋和中印度洋，在近海表和海底发现了1~10 cm2 s−1的二重扩散系数（Kz）。最小值出现在温跃层区域，其中 Kz 降至约 10−2 cm2 s−1。下面，Kz 的值主要在 0.1–1 cm2 s−1 范围内，并向下增加。水平（等密度）扩散率在 104 至 108 cm2 s−1 之间变化，主要在 106–107 cm2 s−1 范围内。上升流速度大多在 1–10 m yr−2 范围的下端，而下降流则与往往是局部的上升流不同，发生的速度为 0.1–1 m yr−2 。地下流的经向分量在0.01~0.1 cm s−1的范围内，大部分向南，并且不限于特定的深度区间。研究区域推导出的海流场提供了从南部引入的深水回流的强度和模式的总体图景，这些深水被认为主要是通过沿盆地西部边界相对狭窄、强烈的水流向北输送的。目前的研究以及 226Ra 高精度和高灵敏度质谱分析的最新进展表明，这种天然存在的同位素值得进一步开发，作为大规模海洋环流和混合过程的速率示踪剂。
Radium 226 and Ba data in the eastern and central Indian Ocean collected during the Geochemical Ocean Sections Study (GEOSECS) expeditions are reviewed and used to delineate the geochemical cycle of both species. The data show that although particulate uptake of 226Ra and Ba occurs in the surface ocean, their regeneration may largely take place near or below the sediment-water interface. Therefore, over much of the deep ocean save close to the bottom, 226Ra can be regarded as being chemically conservative. Taking advantage of this geochemical aspect of 226Ra enables us to compute the vertical (z) and horizontal (meridional, x) eddy diffusivities (Kz and Kx) and advective velocities (Vz and Vx) and their variability in the eastern and central parts of the Indian Ocean. The absolute magnitudes of these mixing parameters are calibrated against the decay rate of 226Ra. The computation is done by numerically solving the mass continuity equations for three properties: 226Ra, total salt, and mass of seawater. The grid size used in the calculations, in which the advective transport is taken to be along the isopycnal surface, has dimensions of Δx ≈ 700 km and Δz ≈ 400 m. The model calculation shows that in the eastern and central Indian Ocean along the GEOSECS traverses, values of diapycnal diffusivity (Kz) of 1–10 cm2 s−1 are found near the surface and the bottom. Minima occur in the thermocline region where Kz falls to about 10−2 cm2 s−1. Below, Kz has values largely in the range 0.1–1 cm2 s−1 and increases downward. The horizontal (isopycnal) diffusivity varies between 104 and 108 cm2 s−1, mainly in the range 106–107 cm2 s−1. Upwelling velocities are mostly in the lower end of the range 1–10 m yr−2, whereas downwelling, unlike upwelling which tends to be localized, occurs with velocities of 0.1–1 m yr−2. The meridional component of subsurface currents is in the range of 0.01–0.1 cm s−1, mostly toward the south, and not restricted to a particular depth interval. The current field deduced in the study area provides a broad picture of the strength and pattern of return flows of the deep water introduced from the south and thought to be principally transported northward via the relatively narrow, intense currents along the western boundaries of the basins. The present study as well as recent developments in the high-precision and high-sensitivity mass spectrometric analysis of 226Ra suggests that this naturally occurring isotope warrants further exploitation as a rate tracer for large-scale ocean circulation and mixing processes.