基于粒级分离的长江口及邻近陆架沉积物磁性特征及其环境意义

Magnetic properties of particle-sized fractions of sediments in the Changjiang Estuary and neighboring shelf,and its environmental implications

在粒级分离的基础上,对长江河口及邻近陆架22个表层沉积物进行了磁学表征,探讨了磁性特征对物源、输运、沉积动力等环境信息的指示意义.研究结果表明,长江入海泥沙、残留砂及废黄河物质是长江口外水下三角洲及邻近陆架沉积物磁性特征的主要物源影响因素,但3者的空间分布不同.沉积物磁化率(c)、饱和等温剩磁(SIRM)、硬剩磁(HIRM)及非磁滞剩磁磁化率(cARM)的空间变化指示了长江入海泥沙出口门后向南及东南方向输运.SIRM与c及退磁参数(S_(-100))与SIRM关系图表明,大于63μm粒级沉积物来自残留砂与现代长江粗颗粒,其分布大致以30 m等深线为界;小于16μm粒级沉积物受长江和废黄河物质的影响,口内为长江源,口外北部贴岸以废黄河源为主,口外其他区域则表现为以长江物质占主导的混合源.沉积物粒度和磁学特征的空间变化反映了动力的粒度和密度分选作用,并体现在沉积环境分区磁性特征差异及分粒级组分对全样SIRM贡献的空间变化上.粒级分离减小了粒度效应对全样磁性特征的干扰,提高了沉积物物源判别的准确性,在反映三角洲地貌变化及物源定量识别上具有重要意义.

Twenty-two surface sediments collected from the Changjiang Estuary and neighboring shelf were subjected to particle-size measurements,with the intent of understanding the implications for provenance,transport,and depositional dynamics.The results showed that Changjiang River-derived sediments,relict sands,and Yellow River-derived sediments were the primary sources controlling the magnetic properties of sediments in the study area.The three areas,however,exhibited different spatial distributions.Spatial variations of magnetic parameters,including magnetic susceptibility(c),saturation isothermal remanent magnetization(SIRM),hard isothermal remanent magnetization(HIRM),and anhysteretic susceptibility(cARM),suggest that sediments from the Changjiang River are transported towards the south and southeast when they move out of the river mouth.According to bi-plots of SIRM versus c and S-ratio(S_(-100))versus SIRM,the>63μm fraction is roughly bounded by the 30 m isobaths that separates the Changjiang River sediment from the relict sands on the shelf.The<16μm fraction is derived mainly from the modern fluvial sources of the Changjiang and Yellow Rivers;in particular,the Changjiang Riverderived sediment dominates the inner estuary and the Yellow River-derived sediment dominates the northern coast of the shelf.The other areas of the shelf are characterized by mixed sources of the<16μm fraction,with a majority being Changjiang River-derived sediment.Spatial variations of particle size compositions and magnetic properties reflect the role of hydrodynamic sorting on particle size as well as mineral density;this results in differences in magnetic properties among the sedimentary units as well as the contribution of different sized fractions to the bulk SIRM values.Particle size separation could reduce the effect of particle size on bulk magnetic properties and lead to more precise provenance discrimination.Our results have great potential in the study of geomorphological changes and quantitative source identification in delta environments.