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. 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 (chi-T curves), hysteresis loops and associated Delta M and d Delta M/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. 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 chi-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 Delta M and d Delta M/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.