北冰洋沉积物中磁性矿物及其剩磁记录机制研究

Paleomagnetic methods had played an important role in constructing age models for Arctic Ocean sediments due to the paucity of fossil records. However, along with the development of research, there are different points of view on the authenticity of negative inclination intervals present in the upper several metres of sediment cores and whether they are records of geomagnetic excursions within the Brunhes or the Matuyama, which result in disputes on the applicability of magnetostratigraphy in the North Pole region. The major cause of this situation is the lack of systematic understanding of magnetic minerals and their remanence recording mechanisms in the sediments. Therefore, five sediment cores with different water depths from different regions of the Arctic Ocean will be investigated in this project. Firstly, systematic rock-magnetic and mineralogical analyses will be conducted to fully clarify the features of magnetic minerals in the studied sediments. Secondly, alternating field and thermal demagnetization will be integrated to isolate different remanent components and to get characteristic remanent magnetization. On the basis of clear understanding of properties of magnetic minerals and along with non-magnetic data, remanence recording mechanisms will then be explored. Last but not the least important, applicability of magnetostratigraphy in construction of chronological frameworks in the studied area will be evaluated entirely. Overall, this project is hopeful to fill the gaps of systematically magnetic research in the North Pole region and to supply important theoretical and methodological guidances to the following relevent studies, which will certainly promote the proceedings on paleoenvironmental investigations.

由于北冰洋沉积物中生物化石稀少，古地磁方法在年龄框架的构建中扮演了重要角色。然而，随着研究的深入，对于岩心上部广泛出现的负极性的真实性及其对应了布容期的极性漂移还是松山负极性期存在不同的观点，以致对磁性地层学在北极地区的适用性产生了争议。造成这种现状的主要原因在于对沉积物中的磁性矿物及其剩磁记录机制缺乏系统的认识。基于此，本项目拟以北冰洋不同海域、不同水深环境的五根沉积岩心为研究对象：首先，通过系统的岩石磁学和矿物学测试分析，充分厘清沉积物中磁性矿物的特征；其次，综合运用交变退磁和热退磁方法，分离不同的剩磁组分并获得原生特征剩磁；再次，在清晰把握磁性矿物性质的基础上，结合非磁学数据，探讨不同剩磁组分的获得机制；最后，全面评价磁性地层学对于研究区沉积地层构建年龄框架的适用性。上述研究有望填补北极地区系统磁学研究的空白，为后续相关研究提供了重要的理论和方法指导，将极大促进该区相关的古环境研究。

近年来，北冰洋深海百万年尺度的磁性地层年代框架受到了来自诸如Mn旋回地层和钛磁赤铁矿“自反转”现象的强烈质疑，以致对磁性地层学方法在该区的适用性产生了巨大的争议。究其根本原因，在于对相应沉积物中的磁性矿物及其剩磁记录机制缺乏系统认识。鉴于此，本项目以历次中国北极科学考察在北冰洋深海获得的5根钻孔岩心(ARC5-ICE4、ARC6-AD04、ARC7-E25、ARC8-LR08、ARC8-GR01)为研究对象，并将ARC5-ICE4孔作为研究重点，开展了深入的岩石磁学和古地磁学以及矿物学测试分析。结果表明，沉积物中的主要载磁组分为磁铁矿，包括碎屑和生物两种成因类型，系生物磁小体在研究区的首次报道；部分碎屑型磁铁矿表面发生了轻微的氧化作用；另含少量磁赤铁矿、赤铁矿、钛磁赤铁矿等；磁性颗粒呈现相对均一的准单畴，且含量随深度的变化不大；ARC5-ICE4孔1.152.04 m区间的负极性并非“自反转”所致，而是真实古地磁场信号的记录。结合相对古强度曲线模式和S-ratio的旋回变化特征，将其对应于松山负极性期(2.580.78 Ma)，钻孔底界年龄< 3.58 Ma。该磁性地层年代框架指示的沉积速率变化模式很好地吻合了北半球冰期加强(iNHG)和中更新世气候转型(MPT)等重要古气候事件，进一步印证了其可靠性。因此，即使钛磁赤铁矿剩磁“自反转”现象在北冰洋深海沉积中真实存在，其影响范围也是局部的。这就说明磁性地层对于建立研究区新生代年代地层框架的重要性仍不容忽视。显然，目前北冰洋深海沉积物定年仍面临诸多难题和挑战，今后需加强多学科协作。此外，基于建立的磁性地层年代框架，项目组成员还开展了新生代以来北冰洋海冰/冰盖演化历史重建及影响机制分析等研究。总之，该项目已完成了预设科学目标，相关研究成果对于准确认识北冰洋深海地区年代地层和古环境演化具有重要的理论和指导意义。