南海南部约80ka以来的古地磁记录

晚更新世以来以一系列地磁漂移事件是区域地层对比和确定沉积物年代框架的重要工具.但是在海相沉积物中,识别这些地磁漂移事件多依赖相对磁场强度(RPI)的变化,在地磁方向异常方面鲜有报到,从而影响对这些事件可靠性的讨论.本文选择南海南部巽他陆架附近重力活塞钻孔BKAS2PC的沉积物,进行系统的岩石磁学和古地磁研究,获得该钻孔沉积记录的RPI和磁倾角变化信息.岩石磁学及扫描电镜结果表明沉积物的原生主要载磁矿物为单畴(SD)和假单畴(PSD)钛磁铁矿.在一些层位,沉积后发生的还原作用形成胶黄铁矿等铁硫化物.磁性矿物的浓度和粒度等参数呈现显著的两阶段变化特征,上部(约220 cm以上)含量低而粒度细,下部含量高但粒度相对较粗,整体上均在一个数量级范围内波动.样品的交变退磁特征显示当交变场超过60 mT时,受胶黄铁矿影响产生旋转磁化,剩磁强度反而上升.因此,在20～60 mT之间确定原生特征剩磁,并把RPI定义为NRM_((20-40)mT)/ARM_((20-40)mT).通过~(14)C限定,并与其他有良好年代控制的相对磁场强度曲线进行对比,建立了钻孔的年代框架.结果显示,钻孔沉积物记录了几次显著的地磁漂移事件,这为联合应用RPI与磁场方向异常构建海相沉积物年代学框架提供了新的依据.

Paleomagnetic records since~80 ka from the southern South China Sea

Geomagnetic excursions during the Late Pleistocene have been widely used for regional stratigraphic correlation and dating. However, the systematic geomagnetic direction anomalies in deep sea sediments are rarely reported because such events are identified mostly by relative paleointensity (RPI). This paper presents the paleomagnetic results of a piston borehole (BKAS2PC) near the Sunda Shelf, southern South China Sea and discusses the inclination variations based on the detailed rock magnetic analysis and RPI. Rock magnetic and scanning electron microscope data show that the original magnetic minerals of sediments are mainly single domain and pseudo-single domain titanomagnetites associated with iron sulfide minerals (such as greigite) that formed during post-depositional diagenetic processes. The concentrations and grain sizes of magnetic minerals display significantly two-phase variations with relatively lower concentrations and coarser grain sizes in the upper part of the core (0~220 cm) while relatively higher concentrations and finer grain sizes in the lower part. The alternating field (AF) demagnetization results suggest that greigite acquired rotational remanent magnetizations at the field above 60 mT and then the remanence intensity of specimen increased. The characteristic remanent magnetization is defined between 20 and 40 mT to avoid effects of iron sulfides. Thus NRM_(20-40)mT/ARM_(20-40)mT is used as the index for relative paleomagnetic intensity. Constrained by ¹⁴C ages, the RPI of the core BKAS2PC is compared with other well dated RPI stacks to establish an age model of the core. Several obvious negative anomalies of magnetic inclination are observed in the borehole and interpreted to be geomagnetic excursions. The combination of RPI and paleomagnetic direction anomalies, therefore, provides a more relaible way for constructing the chronological framework of marine sediments.