Magnetic minerals in sediments of the continental shelf are sensitive to environmental changes.Therefore,to determine the exact phases of magnetic minerals is the prerequisite to investigate the paleoenvironmental significances.In this study,a comprehensive rock magnetic investigation,coupled with mineralogical studies,was carried out on the long core NHH01(with a length of 125.64 m)from the central continental shelf of the South Yellow Sea.Single-domain greigite was identified as the dominant magnetic carrier in sediments between44.90 and 51.80 m,and terrigenous magnetite is the major magnetic phase in the adjacent layers.The existence of greigite unambiguously indicates an anoxic environment.We tentatively interpreted that such an environment could be formed by the fast sea level changes,long-existing of the cold eddies during warm stages,or in a lake which might be related to locally tectonic uplifting.Therefore,the occurrence of the thick-layer greigite could supply a new perspective to paleoenvironmental or even tectonic studies.
Jianxing LiuXuefa ShiShulan GeQingsong LiuZhengquan YaoGang Yang
Magnetite is the most important magnetic mineral in paleomagnetism. Its magnetic properties are controlled by many factors, such as grain size distribution,shape and interactions. Traditional rock magnetic experiments, however, have great difficulty in decoupling the effects of these parameters. In this study, we attempted to investigate the effects of grain size distribution on magnetic properties of magnetite powders by using a micromagnetic method. The particle geometries used in the micromagnetic model were based on the grain size distribution observed in a synthetic magnetite powder. The simulated hysteresis parameters agree well with the experimental measurements and provide clear microstructures of the magnetic remanence. Our results show that grain size plays a more important role in affecting hysteresis parameters of magnetite assemblages than shape under effects of interactions. Uniform or vortex superstates formed by two or more particles are found and display different stabilities of magnetic recording in assemblages.Some domain structures of single-domain(SD) particles are reversed as the applied field decreases to zero. Small pseudo-single-domain particles behave as SD structures and may dominate the magnetic recordings. In all, micromagnetic modeling of grain size distributions provide a better understanding of magnetic assemblages consisting of nanoscale particles.
