还原成岩作用对磁性矿物的影响及古气候意义:以长江口水下三角洲岩芯YD0901沉积物为例

本文对长江口水下三角洲岩芯YD0901(31°11'N,122°30'E)中0~34.22m的沉积物进行高分辨率岩石磁学研究,结果表明: 阶段Ⅰ(0~8m)中磁性矿物含量高,含有磁铁矿、赤铁矿及少量的硫化物; 阶段Ⅱ(8.0~34.2m)中磁性矿物含量低,磁性矿物包括粗颗粒磁铁矿、赤铁矿和铁的硫化物。磁参数结果指示了沉积物垂向上氧化还原环境的改变,由还原环境(阶段Ⅱ)转为氧化环境(阶段Ⅰ)。受成岩作用影响,阶段Ⅱ中的细颗粒的磁铁矿颗粒被溶解而生成铁的硫化物,但是粗颗粒磁铁矿和赤铁矿受成岩作用影响较小,可以提取原生的古气候信息。根据磁性矿物含量和矫顽力的变化在1300年至7000年之间识别出4个气候突然变冷事件,沉积物磁性矿物矫顽力反映的气候变化周期为800年和220年。

DIAGENETIC ALTERATION ON MAGNETIC MINERALS AND THE PALEOCLIMATE IMPLICATIONS,RESULTS FROM CORE YD0901 OF YANGTZE SUBAQUEOUS DELTA

Diagenesis could alter domains,concentration and mineralogy of magnetic minerals,and then destroy the original paleoclimate signal carried by magnetic minerals. It is thus essential to understand the influence of diagenesis on magnetic minerals while using magnetic method to do paleoclimate research.High-resolution magnetic analysis was carried out on U-channel samples from core YD0901(31°11'N,122°30'E;21m water depth;65.23m in length),which was drilled from the Yangtze subaqueous delta.In this study,we mainly discuss the magnetic variations of the uppermost 34.22m mud section part( 63μm grains are less than 2% in volume),with some missing intervals during coring,2.53~2.82m,3.59~3.92m,4.93~5.94m,18.37~21.15m and 21.85~22.20m.Eight 14C AMS dates indicate that the 34.22m mud section spans about the last 7000 years,with average sedimentation rate of nearly 5mm/a.More detailed magnetic analysis(mass susceptibility,hysteresis loop and low temperature MPMS) were also carried out on dried samples at 10cm or 20cm intervals.Magnetic properties vary down-core in two parts,due to post-depositional reductive diagenesis.Section Ⅰ(0~8m) is characterized by high magnetic concentration,which contains magnetite,hematite and some iron sulfide.Section Ⅱ(8.0~34.2m) is characterized by low magnetic concentration,which contains multidomain magnetite,hematite and iron sulfide.The concentration of iron sulfide is higher in section Ⅱ than that in section I.Magnetic variations of this core reflects geochemical and redox environment changes of this area,from reductive to oxic,which can be recognized as sulfidic zone(section Ⅱ)and oxic zone(section Ⅰ).Due to diagenetic alteration,fine PSD and SD magnetite are dissolved and transformed to iron sulfide in section Ⅱ,but there are less influence from diagenesis on coarse PSD and MD magnetite and hematite in this core.Then paleoclimate information is still obtained from these particles,and 4 cold events can be found from magnetic concentration and coercivity.Variation of magnetic coercivity in the sediments has significant power at the periods of 800 years and 220 years.