极地研究

极地研究

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南极布兰斯菲尔德海峡沉积物的地磁场长期变与定年

葛淑兰1,2 陈志华1 刘建兴1,3 黄元辉1 王汝建4 闫仕娟1   

  1.  
    1国家海洋局第一海洋研究所
    2 国土资源部海洋油气资源与环境地质重点实验室
    3 中国科学院地质与地球物理研究所
     4 同济大学海洋地质国家重点实验室
  • 出版日期:2014-03-30 发布日期:2014-03-30

DATING SEDIMENT CORE BY RELATIVE PALEOINTENSITY AND DIRECTION OF GEO,AGNETIC FIELD IN BRANSFIELD STRAIT, ANTARCTIC PENUNSULA

  Ge Shulan1,2，Chen Zhihua1，Liu Jianxing1,3，Huang Yuanhui1，Wang Rujian4，Yan Shijuan1   

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    1 First Institute of Oceanography, State Oceanic Administration, Qingdao
    2 Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources,Qingdao
    3 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
    4 State Key Lab of Marine Geology, Tongji University, Shanghai
  • Online:2014-03-30 Published:2014-03-30

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摘要： 对位于布兰斯菲尔德海峡东北部的沉积物岩心D1-7进行了初步环境磁学和古地磁研究，获得了连续的相对地磁场强度和方向。沉积物岩性总体为灰色软塑性粘土，中下部出现一层黑色火山灰层。结合该孔沉积物和有孔虫AMS14C测年结果（王汝建等未发表资料），相对地磁场强度及其方向提供了12 ka以来连续的定年标尺，其中相对地磁场强度与具有相似沉积速率的南美Laguna Potrok Aike湖泊记录对比提供了六个对比点；同时特征剩磁倾角和磁偏角提供了另外六个对比点。磁化率各向异性分析揭示了全新世早、中和晚期岩心所在位置底流（南极深层水~1000 m）发生了阶段性显著变化，同时磁性矿物含量、粒度、沉积物湿密度等也发生了相应的变化。这些环境磁学和沉积学的变化主要受控于南极相应纬度处的太阳辐射量以及与辐射量相关的夏季季风降水量的变化，降水量增加导致磁性矿物粒度变细。D1-7也记录了一些千年尺度旋回变化，但是南极地区目前缺乏与之相似分辨率的气候参数记录，无法进一步探讨这些气候事件的缘起和分布范围。对比发现，地磁场长期变产生的年龄与沉积物全样有机碳AMS14C测年结果之间存在系统的差异，在6 ka以来比有机碳年龄年轻，在6 ka之前则比有机碳年龄老。南极布兰斯菲尔德海峡内各种不同水体的混合以及在全新世全球升温背景下冰川的动态变化和由此引起的中、深层水变化，都是造成水体性质复杂的原因。而地磁场强度和方向变化则不受水体性质的影响，因此可以提供更合理的年龄信息。自12 ka至今，25 cm/ka的平均沉积速率表明南极布兰斯菲尔德海峡地区整个全新世内稳定和丰富的沉积物供应。

关键词: 南极, 布兰斯菲尔德海峡, 相对地磁场强度, 磁倾角, 地磁场长期变

Abstract: Paleomagnetic and rock magnetic studies are undertaken in sediment core D1-7 of Bransfield Strait, Antarctic, to obtain continuous variation of relative paleointensity (RPI) and direction of geomagnetic field. The lithologies of the core are mainly gray soft and plastic clay, a dark tephra layer at the lower part 2.20-2.21 m with sharp lower contact and gradual upper contact. Combined with AMS14C dating of bulk sediment and foram (Wang R J et al unpublished data), RPI and direction of geomagnetic field provide a continuous time scale of the last 12 ka, among which RPI comparing with Laguna Potrok Aike lake of South America produced six tie points, meanwhile, inclination and declination comparing with this record results in additional six tie points. Drastic changes of bottom current (1000 m, Antarctic deep water) for early, middle and late Holocene are revealed by anisotropy of magnetic susceptibility (AMS), as well as magnetic concentration, grain size and wet density. These changes basically are controlled by summer insolation at 65 ?S, therefore by the summer monsoon precipitation in cycles of precession. Millennial scale variations are also observed but no other records with similar resolution are available to decipher origin and ranges of these climate or environment events. The ages from secular variation are systematically younger/older after/before 6 ka B. P.. The mixing of different water masses and intermediate, deep water evolution related with Antarctic ice sheet retreat during Holocene are reasonable reasons for complex carbon input in Bransfield Strait. The intensity and direction of geomagnetic field recorded by sediments can avoid this influence and provide reliable age information. An average sedimentation rate of 25 cm/ka for the whole Holocene in D1-7 suggest a rich and constant terrigenous input in Bransfield Strait, Antarctic.

Key words: Bransfield Strait, Antarctic, relative paleointensity of geomagnetic field, inclination, secular variation of geomagnetic field