5:15 PM - 6:30 PM
[SEM13-P04] Rock-magnetic properties of topmost sediments in Omura Bay, Nagasaki Prefecture
Keywords:rock-magnetic property, topmost marine sediment, origin of magentic minerals, eary diagenesis
We are analyzing magnetic properties of topmost sediments in Omura Bay, Nagasaki Prefecture, in order to differentiate magnetic signals from magnetic grains of different sources in the sediments. The bay is a highly enclosed bay with the water depth of 15-20m, connecting to East China Sea through two narrow channels. There is no large river loading to the bay. Sediments above 4.5 m below seafloor consists of homogenous silty clay with the medium grain size of 4 micrometer. Severe hypoxia expands every summer in bottom water (July-August). We are also examining the effect of such an anoxic condition in the bottom water on magnetic properties in the topmost sediments.
Sediment cores of about 15 cm long were sampled at the central part of the bay in July and October 2017 by a G.S. type-gravity core sampler. The cores were sliced at 1cm interval, and each slice was freeze-dried. Dry powder samples were packed into 1cc-plastic cubes. The cubes and remaining powder samples were subjected to rock magnetic experiments.
Results from thermo-magnetic analyses with magnetic balance and progressive thermal demagnetization experiments of composite isothermal remanences (IRMs) indicate the existence of magnetite, maghematized magnetite and hematite as principal magnetic minerals. Values of initial magnetic susceptibility, saturation magnetization and saturation remanent magnetization show no remarkable change through the cores. Downcore increase of the susceptibility of anhysteretic remanence and S-ratio (S_-0.1T) and downcore increase of the coercivity of remanence are detected. Results from coercivity distribution analyses of IRM acquisition and demagnetization curves suggest that the contribution of high-coercivity magnetic grain, probably hematite, to the whole of IRM increase downcore although its amount does not change. It may be implied that a downward selective dissolution of fine-grained magnetite by the effect of early diagenesis in the topmost sediments results in downward increases of magnetic grain size and contribution (relative amount) of hematite. We have not found any difference in magnetic properties and their variations mentioned above between the two cores of different month.
Sediment cores of about 15 cm long were sampled at the central part of the bay in July and October 2017 by a G.S. type-gravity core sampler. The cores were sliced at 1cm interval, and each slice was freeze-dried. Dry powder samples were packed into 1cc-plastic cubes. The cubes and remaining powder samples were subjected to rock magnetic experiments.
Results from thermo-magnetic analyses with magnetic balance and progressive thermal demagnetization experiments of composite isothermal remanences (IRMs) indicate the existence of magnetite, maghematized magnetite and hematite as principal magnetic minerals. Values of initial magnetic susceptibility, saturation magnetization and saturation remanent magnetization show no remarkable change through the cores. Downcore increase of the susceptibility of anhysteretic remanence and S-ratio (S_-0.1T) and downcore increase of the coercivity of remanence are detected. Results from coercivity distribution analyses of IRM acquisition and demagnetization curves suggest that the contribution of high-coercivity magnetic grain, probably hematite, to the whole of IRM increase downcore although its amount does not change. It may be implied that a downward selective dissolution of fine-grained magnetite by the effect of early diagenesis in the topmost sediments results in downward increases of magnetic grain size and contribution (relative amount) of hematite. We have not found any difference in magnetic properties and their variations mentioned above between the two cores of different month.