Magnetic Properties Of Sediments From The Okinawa Trough And Their Relationships To Sedimentary Environment

Multiple experiments such as grain-sized separation , magnetic mineral analysis, magnetic parameters measurement were made on 110 surface samples and 64 samples of the 180 core sediment taken from the shelf of the East China Sea to abyssal plain of the Okinawa Trough. The magnetic parameters measurement include magnetic susceptibility, saturation remanent magnetization, x(T) curve , hesteresis loop, anhysteresis remanent magnetazation and so on. Based on the measured results, some science problems such as the contribution of the grain size to the magnetic susceptibility, correlation in sediment magnetic characteristic and material provenance with different sedimentary environments, magnetic exhibition of plaoleoceanography event were discussed. The main results and conclusions are presented as following:1 The properties of hesteresis loop, isothermal remanent magnetization and x(T) curve show that the magnetic mineral is dominated by magnetite with a small amount of hematite.The granulometry of magnetic fractions in samples is primarily in pseudo-single-domain state, possibly existing a small amount of superparamagnetic (sp) particles.2 The magnetic minerals exist as different grain-sized particles that contribute magnetic susceptibility in different proportion to bulk sediments from different sedimentary environment. For example, the coarse grain-sized particles contribute much magnetic susceptibility to bulk sediments from the continental shelf edge of the East China Sea, while the fine grain-sized particles contribute much magnetic susceptibility to bulk sediments from the continental slope of the East China Sea and western abyssal plain of the Okinawa Trough. The sediments from eastern abyssal plain of the Okinawa Trough contain some volcanic materials and thus their totalmagnetic susceptibility is both from coarse and fine grain-sized particles.3 According to grain-size contribution to total magnetic susceptibility of bulk sediments, magnetic mineral, magnetic domain state and hystersis properties and the width and height of magnetic parameters curve changes different degree and trend in different sediment environment. That indicates that there are different provenance and formation in different sediment environments. It is concluded that sediments from different environments, continental slope of the East China Sea, western and eastern abyssal plain of the Okinawa Trough, are correlated each other in material source. Other local environment factors, such as turbidity current and volcanic activity, have also impacted on sediments and made them showing different magnetic properties and environmental characteristics.4 According of east abyssal plain of the Okinawa Trough with some volcanic materials, it show strong magnetic characteristics different from other sediment environments.5 There are no evidently correlation between CaCO3 and magnetic parameters, but magnetic feature tends to become weaker with the content of CaCO3 increase. Being as biologic index, CaCO3 possibly dilute the content of magnetic minerals of sediments to affect on magnetic characteristics of sediments .6 It have verified that two Volcano Ejecting events and Younger Dryas event had happened in the Okinawa Trough from the magnetic information contained in sediments of the typical core. The time of Volcano Ejecting events happened at 12.6ka and 6.2ka respectively, and the time of the last Volcano Ejecting event is consistent with K-Ah Volcano Ejecting events. Where the Younger Dryas event happened in 180 core, the magnetic susceptibility curve markedly decrease. At the same time SIRM/x and ARM/x curve little increase. It record the characteristic of environmental magnetism at the Younger Dryas event.