黄河三角洲及邻近海域沉积物137Cs和210Pbex断代的适用性

¹³⁷Cs和²¹⁰Pb_ex定年是确定近百年来浅层湖泊沉积物年代学框架的首选方法,然而,该方法在河口滩地及邻近海域沉积物断代中的适用性仍然不清。本文以渤海黄河三角洲为研究对象,通过钻孔采集了1929～1990年间四个不同成陆年限的淤成土泥芯,分析了¹³⁷Cs和²¹⁰Pb_ex活度及粒度的深度分布特征;结合已报道的邻近海域水下地形特征、沉积物特性、沉积构造等资料,讨论了黄河口泥沙沉积和三角洲形成的特点,评价了沉积物¹³⁷Cs和²¹⁰Pb_ex断代法在该区域的适用性。结果表明,黄河三角洲是由入海口冲积扇不断向前推进而形成,具有快速且不均匀沉积的特点。20世纪50～70年代大气层核试验产生的放射性同位素¹³⁷Cs是黄河三角洲和邻近海域沉积物断代可靠的示踪物,含¹³⁷Cs的沉积物可被确定为1963年(或1954年)以来的沉积物。²¹⁰Pb_ex是大气持续沉降的天然放射性尘埃,不适用于具有快速且不均匀沉积特点的入海口水下冲积扇沉积物断代;水下冲积扇成陆后,地面继续承接²¹⁰Pb_ex核数沉降,因此,²¹⁰Pb_ex也不适用于黄河陆地三角洲的沉积物断代。泥沙的沉积速率在远离水下冲积扇的海域较低,且经水面沉降至海底的²¹⁰Pb_ex比例较高,沉积物的²¹⁰Pb_ex深度分布曲线形态类似于稳定环境的湖泊、海湾。计算沉积速率的CIC和CRS模型未考虑核素的向下扩散迁移过程,这可能会导致估算的黄河三角洲邻近海域泥沙沉积速率偏大。

Suitability of the 137Cs and 210Pbex Techniques for Dating of Sediments in the Yellow River Delta and the Aadjacent Sea Area

¹³⁷Cs and ²¹⁰Pb_ex dating are the important methods to determine the chronological framework of shallow lake sediments deposited over the past 100 years. However, the applicability of these two methods in the sediment dating on estuary mudflats and its adjacent waters is still unclear. In the present study, four sediment cores were collected from the Yellow River Delta (YRD) with different forming years (1929-1990), and the depth distribution patterns of ¹³⁷Cs and ²¹⁰Pb_ex activity and particles composition were analyzed. The characteristics of sediment deposition and delta formation of the YRD have been discussed according to the reported subaqueous topographic features of adjacent waters, sediment properties, and sedimentary structures, finally, the applicability of the ¹³⁷Cs and ²¹⁰Pb_ex dating methods in this area are evaluated. The results show that the YRD is formed by the continuous advancement of alluvial fans at the estuary and characterized by rapid and uneven deposition. The radioactive isotope ¹³⁷Cs produced by atmospheric nuclear tests in the 1950s and 1970s is a reliable tracer for the sediment dating in the YRD and its adjacent waters. The subsurface sediments containing ¹³⁷Cs can be identified as sediments since 1963 (or 1954). ²¹⁰Pb_ex is a natural radioactive dust in the atmosphere that deposited continuously, and it is not suitable for the sediment dating of the subaqueous alluvial fans at the estuary with the characteristics of rapid and uneven deposition. After the underwater alluvial fans become lands, the ground continues to accept ²¹⁰Pb_ex dust deposition. Therefore, the ²¹⁰Pb_ex is also not applicable to the sediment dating in the YRD. The deposition rate of sediment is lower in the waters far away from the subaqueous alluvial fans, and the proportion of ²¹⁰Pb_ex that sinks to the bottom of the water is high. The depth distribution curve of ²¹⁰Pb_ex is similar to that of lakes and bays with stable environment. The CIC and CRS models for calculating the deposition rate neglected the downward diffusion and migration process of nuclides, which may lead to an overestimation of sediment deposition rate in the adjacent waters of the YRD.