南海北部陆坡GHE24L柱样沉积物磁性特征及其环境意义

ENVIRONMENTAL CHANGES AS RECORDED BY MINERAL MAGNETIC PROPERTIES OF SEDIMENTS FROM THE CORE GHE24L,SOUTH CHINA SEA

对取自南海北部陆坡神狐海域的近6m长的GHE24L柱状样沉积物磁性特征的详细研究,探讨了其磁性特征的纵向变化与沉积环境演变的关系。研究结果表明,该柱状样沉积物中的磁性矿物没有明显受到后期还原成岩作用的影响。沉积物中的磁性矿物主要是假单畴磁铁矿,其含量和粒度的变化记录了过去20ka以来的环境演化,尤其是对末次盛冰期以来的几个重要气候事件如Heinrich 1事件、Bolling-Allerod暖期、新仙女木事件等有很好的响应。研究结果可为重建南海区域环境演化提供依据,也可为探究东亚海陆气候变迁的耦合联系提供约束。

Lots of achievements about the paleoceanographic and paleoenvironmental evolution of the South China Sea （SCS） have been obtained on the glacial-interglacial time scales using multiple proxies including stable isotopes, spores, pollen and so on. Magnetic proxies have not been used extensively on studying the SCS sediments, despite that it has been demonstrated to be a very useful tool in reconstructing the evolution of the SCS. To better understand the environmental significance of magnetic properties of sediments from the SCS, we have conducted a detailed environmental magnetic study of sediments from the Core GHE24L, which was retrieved from water depth 1387m from the Shenhu Water of the northern SCS. It is 5.96m long and consists of light grey mud and silty mud. After the core was split, a total of 286 samples were collected every 2cm from the working half using 8-cm^3 plastic cubes. A number of magnetic parameters have been measured for these samples including magnetic susceptibility （Z） , anhysteretic remanent magnetization （ ARM）, isothermal remanent magnetization （IRM） acquisition curve, saturation IRM （SIRM） , magnetic hysteresis and so on. The results show that Z varies from 19.47x10^-8m^3/kg to 33.84x 10^-8m^3/kg, XARM ranges from 40. 56X 10^-7m^3/kg to 59.44x10^-7m^3/kg, and that SIRM range is between 27.35x10^-4Am^2/kg and 50.05x10^-4Am^2/kg, S-ratio is close to 1. These data indicate that pseudo-single domain magnetite grains dominate the sediments and sediments appear to have faithfully recorded environmental changes of the study area for the past ca.20ka. Changes in the content and grain size of magnetic minerals of the sediments were mainly controlled by the terrigenous input, which is in turn governed by climate changes by modulating weathering processes. By comparing the variations of different magnetic parameters of the Core GHE24L with the δ18O of the nearby Core 17940, which is very well-dated and deemed as a reference for the northern SCS, we found that changes in magnetic characteristics of the sediments record three distinct stages of evolution. Stage I, from 20. 80ka to 18.74ka, is characterized by high Z, SIRM, and low XARM/X and XARM/SIRM, indicating enhanced terrigenous input and the dominance of coarser component of magnetic grains during the LGM. Stage Ⅱ, from 18.74ka to ll. 31ka, is characterized by fluctuations of various magnetic parameters, particularly those indicative of grain size variations. These magnetic parameters recorded major climate shifts of this region including the Heinrich 1 event, Bφblling-Allerφd warm event, Younger Dryas event during Stage Ⅱ. StageⅢ, from 11.31ka to 0. 36ka, recorded strengthening of Asian Summer Monsoon during early Holocene and the surface circulation in South China Sea, which may have carried coarse sediments from offshore Taiwan to the northern slope of the South China Sea, leading to gradually coarsening upward during the Mid to Late Holocene. This study not only provides a basis for the SCS＇ paleoenvironmental reconstruction from Last Glacial Maximum, but also can provide some constraints for exploring the coupling link about land-sea interactions of the Earth system.