Magnetostratigraphy Of Site U1505 Of IODP Expedition 368 In The South China Sea During The Late Miocene To Pliocene

The South China Sea?SCS?is situated at the junction of the Eurasian,Pacific,and Indo-Australian plates,and is affected by the East Asian monsoon.Its sediments,which have preserved the geological records of plate movement and climate fluctuation since Cenozoic,could be an excellent material to study the evolution history of the marginal basin and reconstruct the paleoenvironment in Western Pacific.For the purpose of examining the fault pattern in the northern margin of the SCS,studying the tectonic events and basin evolution since Eocene as well as Neogene paleocean and paleoclimate changes,a series of deep-penetration sites were drilled within a Continent-Ocean-Transition?COT?zone during International Ocean Discovery Program?IODP?Expedition 368.In order to establish a reliable chronological framework of the study area,detailed rock magnetism and paleomagnetism experiments were carried out on 280 discrete samples taken from⁶0-200 mcd cores of Hole U1505C and U1505D.Rock magnetic results from temperature dependence magnetic susceptibility??-T?,isothermal remanent magnetization?IRM?acquisition,hysteresis loop parameters,CLG model of IRM curves and Day plot indicate that the main magnetic mineral is low coercivity magnetite,and a small amount of hematite is also contained.Only few samples contain greigite.The anisotropy of magnetic susceptibility?AMS?fabric shows a strongly oblate forms,which consists with the typical feature of lacustrine or deep-sea sediments.The minimum?k₃?susceptibility axes of most samples are perpendicular to the bedding plane,while the maximum?k₁?and the medium?k₂?susceptibility axes lie in the plane,which indicate stable sedimentary environment.Through the stepwise alternating field demagnetization?AFD?with a maximum field of 80-140 mT or thermal demagnetization?TD?with a maximum temperature of600?,the characteristic remanent magnetization?ChRM?with moderate downward or upward inclinations was identified clearly after removing a low-stability drilling-induced remanence with steep downward inclinations.Nine positive and nine negative zone or subzone were identified from the sifted data.The mean sediment accumulation rates for upper section?⁶2.4-79.5 mcd?,mediate section?⁷9.5-106.1 mcd?and lower section?¹11-186.9 mcd?of the core are calculated as¹0.33 m/Myr,²5.725 m/Myr and¹6.34 m/Myr,respectively.Based on the reflection of magnetic parameters to the changes of sedimentary environment,we inferred that the acceleration of sedimentary rate in mediate section is related to the enhancement of erosion in Taiwan region during⁵-4 Ma.By correlating the magnetic polarity sequence to the Geomagnetic Polarity Time Scale 2016 in combination with biostratigraphic data,a new magnetostratigraphic time framework was established.