[S-EM16] Geomagnetism, paleomagnetism, and rock magnetism

[SEM16-P12] Highly magnetic intensity carbonate crusts collected from cold seep sites in the northern Okinawa Trough

*Noriko Kawamura¹, Tomo Aoki² (1.Japan Coast Guard Academy, 2.Japan Coast Guard)

Keywords:carbonate crust, biogenic magnetite, biogenic greigite

The Okinawa Trough is located on the western Pacific Ocean and is an active back-arc basin. The sediment and carbonate crust samples were collected from cold seep in the northern Okinawa Trough. Rock magnetic parameters of the samples; low-field magnetic susceptibility, anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) were measured. IRM component analysis and thermomagnetometry were also conducted. Thermomagnetometry results suggest that magnetic carriers of the sediments are magnetite (Fe₃O₄), greigite (Fe₃S₄) and/or pyrrhotite (Fe₇S₈). Results of X-ray diffraction spectrums indicate that calcite is a main mineral in carbonate crust samples. Although calcite is a diamagnetic mineral, magnetic susceptibility of all carbonate crusts indicates positive values. The thin sections of crust samples were observed using a polarizing microscope, the diffusions of terrestrial materials and pyrite were recognized. The presence of a clear Verwey transition and low coercivity components were recognized in carbonate crusts. It implies that a magnetic carrier of carbonate crusts is biogenic magnetite and/or greigite in terrestrial materials.