长江三角洲全新世地层中潮滩沉积磁性特征及其古环境意义

Magnetic Properties of Holocene Tidal Flats in the Yangtze Delta and Their Paleoenvironmental Implications

对长江三角洲北翼江苏南通地区NT钻孔（长60.9 m）进行了系统的环境磁学分析,并结合岩性特征、粒度、漫反射光谱（DRS）等手段,探讨了全新世早、晚期潮滩沉积的磁性特征及其古环境意义。NT孔自下而上可分为6层（U1~U6层）,其中U2层下部（49.9~44.8 m）和U6层（7.5~0.3 m）为潮滩沉积,具有较低的退磁参数S比值及较高的硬剩磁（HIRM）和SIRM/χ,表明反铁磁性矿物如赤铁矿、针铁矿等含量和比例较高。结合漫反射光谱（DRS）分析,发现U6层上部盐沼（1.5~0.3 m）赤铁矿和针铁矿富集,U2层下部的盐沼仅富集赤铁矿。这一差异与U2层和U6层形成的时期和沉积环境有关。U2层形成于晚更新世晚期至早全新世,且曾长期暴露地表,有利于赤铁矿的形成,其后随着海平面的持续上升,盐沼不断垂向加积,始终处于水下环境,不利于针铁矿的形成;U6层形成于晚全新世三角洲海岸的进积过程中,氧化还原相互交替的环境有利于针铁矿的形成,后期成陆后的成土作用生成了较多的磁赤铁矿和赤铁矿。研究表明,全新世三角洲发育过程中,不同时期形成的盐沼具有不同的磁性特征,磁性特征的研究可以提供潮滩沉积环境演变的信息,对三角洲古环境重建研究具有重要意义。

In this paper,a core（ NT,60.9 m in length） in the northern part of the Yangtze River Delta,from Nantong,Jiangsu province,were subjected to environmental magnetic analysis. In combination with analysis of lithology,particle size and diffuse reflectance spectroscopy（ DRS）,this paper discusses the magnetic properties of tidal deposits formed in early and late Holocene and its paleoenvironmental implications. Core NT can be divided into six depositional units,namely U1 to U6,from bottom to top,among which the lower part of U2（ 49.9 ~ 44.8 m） and U6（ 7.5 ~0.3 m） are interpreted to be tidal flat facies. They are characterized with lower S-ratios and higher hard isothermal remanent magnetization（ HIRM） and SIRM / χ values,which indicates higher concentration and proportion of antiferromagntic minerals such as hematite and goethite. In combination with DRS,it is found that the enriched hematite and goethite occur in the upper part of U6,while only hematite is enriched in the lower part of U2. Such a difference in magnetic mineralogy is related to the sedimentary environments when these sediments are deposited. Sediments in U2 were formed during late stage of late Pleistocene to early Holocene and has been in subaerial condition for a long period,which favors hematite formation and preservation. With the rising sea-level during early Holocene,the salt marsh accreted vertically and eventually inundated,which does not favor the formation of goethite. In contrast,sediments in U6 were formed during the late Holocene. The oscillating redox condition favors the formation of goethite. Abundant maghemite and hematite were formed due to pedogenesis after land formation at a later stage. This study shows that sediments in salt marsh formed in different stages of the Holocene exhibits different magnetic properties. As a result,magnetic properties of sediments can provide information about the evolution of tidal flat,which has important implication for delta paleoenvironmental reconstruction.