长江口水下三角洲Y7柱样磁性特征及其影响因素

MAGNETIC PROPERTIES OF CORE Y7 FROM SUBAQUEOUS DELTA OF THE CHANGJIANG ESTUARY AND THEIR INFLUENCING FACTORS

以长江口水下三角洲380cm长的Y7柱样为对象,在磁性测量、地球化学和粒度分析的基础上,探讨了近百年以来长江口水下三角洲沉积物磁性特征变化及其影响因素。研究结果表明,磁铁矿是主导Y7柱样磁性特征的主要矿物,相比常量元素和粒度的均一组成,磁性特征存在显著的垂向变化。220cm以下,频率磁化率(fd)、非磁滞剩磁磁化率(ARM)、磁化率()、饱和等温剩磁(SIRM)、ARM/以及ARM/SIRM随深度增加而下降,而退磁参数S-100mT和S-300mT则呈相反变化趋势;220cm以上,fd,,ARM/,ARM/SIRM以及S-100mT和S-300mT随深度变化不大,而SIRM和ARM则随深度增加呈现下降趋势。Y7柱样中硬剩磁(HIRM)随深度变化不大。结合Mn/Fe比值的变化,研究认为Y7柱样300cm以下层位,随深度增加沉积物磁性减弱和超顺磁颗粒急剧减少,还原成岩作用是重要因素之一,不完整反铁磁性矿物因相对稳定而导致其在磁性矿物组合中的比例上升。此外,长江中上游主要支流泥沙贡献率的变化可能会对Y7柱样磁性特征有一定影响,但尚需进一步的工作支持。

The Changjiang River has a catchment area of about 1.8 ×10^6km^2 with the population of more than 4 billion. The impact of environmental changes driven by global climate changes and intensive human activities on the estuary of the Changjiang River has drawn great attentions worldwide. Estuarine delta deposits can provide high-resolution sedimentary record for environmental changes due to its high sedimentation rate. In this study, we offer a preliminary research on magnetic record of delta deposit from the Changjiang Estuary, with the purpose to provide a basis for environmental change reconstructions using environmental magnetic approach. Core Y7 （31°N, 122.7°E ） was obtained from the prodelta region of the subaqueous delta of the Changjiang Estuary, China. This core is ca. 380cm in length, which was deposited within the last 100 years based on ^210Pb and ^137Cs datings. Based on magnetic measurements, geochemical and particle size analyses on core Y7, this paper discusses the variations of sedimentary magnetic properties and their influencing factors, such as early diagenesis, sediment provenance, and particle size. The results indicate that magnetite dominates magnetic properties of core YT. In comparison to the uniform compositions of major elements and particle size, magnetic properties display significant down core variations. Below 220cm, frequency-dependent susceptibility （ χfd ）, susceptibility of Anhysteretic Remanent Magnetization （ χARM ）, magnetic susceptibility （χ）, Saturation Isothermal Remanent Magnetization （ SIRM ） and ratios χARM/% and χARM/SIRM decline with depth, while demagnetization parameters S-100mT and S-300mT display an opposite trend. Above 220cm, magnetic parameters χfd,χ,χARM/χ,χARM/SIRM, S-100mT and S-300mT show little variations,while SIRM and χARM decline with depth slightly. Throughout the core, Hard Isothermal Remanent Magnetization （HIRM） display little variation. In combination with the variation of Mn/Fe ratio, it is suggested that reductive diagenesis leads to the decrease of ferrimagnetic minerals, especially fine viscous superparamagnetic grains, in sediments below the 300cm depth. In addition, variations in sediment contributions from different tributaries in the upper and middle sections of the Changjiang River may have played a role in the changes of magnetic properties with depth. It is well established that the volume of sediment delivered to the estuary has changed significantly during the last 50 years, and the contributions of sediment discharges from the major tributaries, namely the Jinsha River, Jialing River, and Han River, are also changing due to various factors, including dam constructions, forestation, and deforestation in the catchments. Further magnetic study of sediments from the tributaries of the Changjiang River is required to evaluate such a suggestion. This study demonstrates that environmental magnetic properties are quite sensitive to environmental changes, and the elucidation of the underlying mechanisms will contribute greatly to environmental change reconstructions using sedimentary records.