长江河口水下三角洲^(210)Pb分布特征及其沉积速率

Sedimentation Rates and Characteristics of Radionuclide ^(210)Pb at the Subaqueous Delta in Changjiang Estuary

通过对长江水下三角洲采集的十个沉积物柱状样中的放射性核素210Pb分析得知,长江水下三角洲表层210Pb放射性比度在2.15～4.22dpm.g-1之间,210Pb沉降通量在1.50～11.21dpm.cm-2yr-1之间,过剩210Pb总量在>48.29～>361.68dpm.cm-2之间;210Pb沉降通量以及过剩210Pb总量均高于理论值,这表明在沉积物中存在210Pb的净输入和聚集。由210Pb放射性比度剖面所反映的沉积速率可知长江水下三角洲泥质沉积区沉积速率介于1.36～4.11cm.yr-1之间;总体上呈现近岸沉积速率较低,沿31°N纬线向20m等深线沉积速率升高的趋势,从122°15′E到122°30′E范围为长江水下三角洲的泥质沉积物堆积中心,其平均沉积速率为3.51cm.yr-1。

The 10 sediment cores were collected from subaqueous delta of Changjiang estuary in April 2006, using the gravity coring equipment. The length of the cores varied from 100 to 160cm. Following visual inspection for geological parameters and photography of the cores, they were subsampled at 2 cm intervals for the entire length of the cores. Profiles of 210pb measured in 10 sediment cores were analyzed to elucidate the sediment rates as well as the characteristics of 210Pb. 210Pb （via 210Po） was determined by α-spectrometry. Distribution of sedimentation rates reveals that during the last 100 years, the derived sedimentation rates vary from 1.36 to 4.11 cm · yr^-1, and generally increase seaward the 20m isobaths. In addition, the region from 122°15′E to 122°30′E, i.e. from cores SC05 to SCll, is considered as the mud depocenter in Changjiang estuary, with the mean sediment rates of 3.51cm · yr^-1. This pattern could coincide with the dispersal of Changjiang River-derived sediments, with influence by the current and tidal systems. The 210Pb activities of the surfacial sediment vary from 2.15 to 4.22dpm · g^-1, and 210Pb deposition fluxes from 1.50 to 11.21dpm · cm^-2 · yr^-1, as well as excess 210Pb inventories between 〉 48.29 and 〉 361.68dpm · cm^-2 , respectively. The excess inventories appear to be higher than the theoretical inventory at the subaqueous delta of Changjiang estuary. It is indicated that besides the atmospheric 210Pb fallout, the fluvial inputs could play an important role, whilst the much riverine sediment discharges may be scavenged and deposited at the subaqueous delta of Changjiang estuary.