渤海南部莱州湾Lz908孔沉积物的岩石磁学性质

亚洲大陆边缘海和陆表海在区域的物质和能量交换以及区域气候与环境演化过程中扮演了关键角色.磁性地层学和环境磁学方法是建立年代框架和环境演变序列的有效手段,但是,由于该地区边缘海和陆表海沉积物中磁性矿物来源十分复杂,磁性地层学和环境磁学研究的重要基础是要精细地解译沉积物的岩石磁学性质.为此,本文利用渤海南部莱州湾Lz908孔与钻孔附近的现代沉积物样品进行了详细的岩石磁学对比研究.结果显示,渤海南部沉积物中的磁性矿物主要是较粗颗粒(较大的准单畴至多畴)磁铁矿,还有少量磁赤铁矿,部分沉积物还含有赤铁矿和针铁矿,其中磁铁矿是特征剩磁的主要载体;莱州湾现代河流-海洋沉积物和钻孔样品之间的磁性特征无显著差异,说明莱州湾沉积物堆积之后尚未经历明显的沉积后期改造.

Rock magnetic properties of the Lz908 borehole sediments from the southern Bohai Sea,eastern China

Marginal seas and epicontinental seas in eastern Asia play an important role in moderating material and energy flux linkages between Asia and Northwest Pacific and subsequently have a profound climatic and environmental impact. Magnetostratigraphy and environmental magnetism have been proven to be useful in the establishment of chronostratigraphic framework and paleoenvironmental processes, respectively. However, due to the complexities of magnetic mineralogy, magnetic grain size and magnetic concentration for the sediments in marginal and epicontinental seas of eastern Asia, it is necessary to carry out detailed rock magnetic studies using multiple parameters before magnetostratigraphic and enviromagnetic analyses.#br#The Bohai Sea is part of the epicontinental seas in eastern China, and the Bohai Basin is one of the Cenozoic extensional basins in this area. In this study, we have carried out detailed rock magnetic investigations on the sediments from the Lz908 borehole sedimentary sequence of the southern Bohai Sea and the modern fluvial/marine sediments from the adjacent areas. The Lz908 core was drilled to a depth of 101.3 m below the surface. The upper 54.3 m of the core contains dominantly marine and coastal silts, sandy silts and fine-grained sands, and the lower 47.0 m consists mainly of lacustrine and fluvial silts. The rock magnetic measurements include temperature-dependent magnetic susceptibility (-T curves), hysteresis loops and associated ΔM and dΔM/dB curves, isothermal remanent magnetization (IRM) acquisition curves and backfield curves of IRM, first order reversal curve (FORC) diagrams, and stepwise thermal and alternating field demagnetization.#br#The multiparameter rock magnetic measurements indicate that the sediments consist of a mixture of different magnetic minerals with variable grain size or concentration. The behaviors of -T curves suggest the presence of magnetite and maghemite in the sediments and the dominant contribution to susceptibility by magnetite. The combination of hysteresis loops and associated ΔM and dΔM/dB curves, IRM acquisition curves and backfield curves of IRM, and stepwise demagnetization suggest the dominance of low-coercivity magnetic minerals, such as magnetite and/or maghemite. High-coercivity magnetic minerals, e.g., hematite and/or goethite are also present, but do not dominate the magnetic signals. The Day plot and FORC diagrams indicate that the dominant ferrimagnetic mineral (magnetite) is of relatively coarse-grained nature, that is, large pseudo-single domain to multidomain-like. Magnetite is the dominant carrier of the characteristic remanent magnetizations (ChRMs). There is no remarkable difference about rock magnetic properties between modern fluvial/marine and borehole sediments, suggesting negligible post-depositional reworking.