Lead 210 and silicate profiles from sediments of the equatorial Pacific and South Atlantic

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

本研究通过过剩铅-210（excess ²¹⁰Pb）与硅-32（³²Si）的测量，确定了5个南大西洋/南极深海沉积物岩芯与3个赤道太平洋深海沉积物岩芯的颗粒混合速率。针对这些硅质、钙质及富黏土沉积物的放射性核素剖面，本研究采用稳态垂直平流扩散模型开展了评估分析。
在南极硅质沉积物中，铅-210（²¹⁰Pb）混合系数（0.04~0.16 cm²/年）与硅-32（³²Si）混合系数（0.2或0.4 cm²/年，具体取决于硅-32的半衰期）处于合理的吻合区间内。但在一处赤道太平洋沉积物岩芯中，铅-210混合系数（0.22 cm²/年）是硅-32混合系数（0.03或0.07 cm²/年）的3~7倍。
太平洋沉积物中铅-210与硅-32混合速率的差异可归因于以下三点：(1) 混合过程并非稳态，且两种放射性核素的特征时间与深度尺度存在差异；(2) 相较于携带大部分硅-32活度的粗颗粒（大型放射虫），富含多数铅-210活度的细粒黏土颗粒更易发生选择性混合；(3) 锰结核底部释放的氡-222（²²²Rn）会提升深部沉积物中测得的过剩铅-210活度（相对于镭-226（²²⁶Ra）），进而人为抬高铅-210的混合系数。
基于硅-32数据与孔隙水硅剖面的分析结果，沉积物柱中生源硅的溶解作用对混合层内的硅-32剖面影响较小。
本研究及已发表文献中报道的深海颗粒混合速率，均与沉积物类型、沉积物堆积速率或表层生产力无显著相关性。通过对彼此间距不足50 km的3个南极岩芯的混合速率差异分析可知，深海混合强度的局地变异，与沉积物属性的区域差异对混合速率的影响程度相当。