南黄海北部末次冰期棕黄色细粒沉积物的磁学特征及其地质意义

MAGNETIC PROPERTIES OF THE LAST GLACIAL BROWNYELLOWFINEGRAINED SEDIMENT IN THE NORTHERN SOUTH YELLOW SEA:IMPLICATION FOR ITS ORIGIN

南黄海北部YA023柱状样0.24～1.45m为末次冰期棕黄色细粒沉积物,以往的研究认为它属于风成黄土沉积。本文对这段沉积物的磁学性质进行了研究,包括磁化率(K)、非磁滞剩磁(ARM)、等温剩磁(IRM)、剩磁矫顽力(Hcr)、低/高矫顽力磁性矿物的比例(S300)、三轴等温磁化实验和磁组构等。实验结果表明,该沉积物的磁性矿物以赤铁矿为主、磁铁矿或钛磁铁矿的含量很低;磁化率各向异性度(P)与磁面理(F)不相关,而P与磁线理(L)存在一定的相关性,磁化率椭球体既有拉长型(扁率E<1)、也有压扁型(E>1),这些实验结果与中国风成黄土的磁学特征显著不同。我们认为,YA023柱状样0.24～1.45m的细粒沉积物应属于末次冰期洪泛沉积,洪泛平原或河漫滩由于周期性地出露地表和被洪水淹没,使沉积物发生周期性成壤化作用和还原成岩作用,导致赤铁矿含量的增加和低矫顽力磁性矿物被溶解。

The last glacial finegrained brownyellow sediment recovered in the 0.24 to 145 m of Core YA023 was regarded as eolian loess deposit in the past researches This paper presents research results of magnetic properties of the sediment, including magnetic susceptibility (K), anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), coercivities of SIRM (Hcr), and S ratios (relative abundance of lowcoercivity magnetic minerals), thermal demagnetization of a threecomponent IRM and anisotropy of magnetic susceptibility (AMS). The results indicate that the magnetic mineralogy of the sediment is dominated by hematite with minor amount of magnetite and titanomagnetite, and that the magnetic anisotropy degree (P) shows no relationship with the magnetic foliation (F) but has a certain correlation with the magnetic lineation (L), with ellipsoid shapes of magnetic susceptibility ranging between elongate and flat. These magnetic properties are remarkably different from those of eolian loess. We put forward that the finegrained sediment in 024 to 145 m of Core YA023 was formed as floodplain deposit, which, due to periodic subaerial exposure and flood submergence, was subject to periodic pedogenesis and reductive magnetic diagenesis, leading to increase of hematite and dissolution of lowcoercivity magnetic minerals such as magnetite and titanomagnetite.