Magnetostratigraphic Study Of The Site U1501,IODP368 In The South China Sea Since The Late Miocene

The South China Sea is the largest marginal sea on the edge of the Western Pacific Ocean,located at the junction of the Eurasian plate,the Pacific plate and the India-Australia plate.The South China Sea is located low latitudes with about 3.5million square meters and a maximum water depth of more than 5,500 meters.Its rich sediments record global or regional paleoclimate and paleoenvironment signals.It is an ideal place for deep-sea research.International Ocean Discovery Program?IODP?367/368 expeditions in 2017 drilled cores in the South China Sea.In this paper,magnetostratigraphic study was carried out on the core of borehole C in position U1501 of 368 expeditions.Through systematic rock magnetism and paleomagnetism experiments,the high resolution magnetic stratigraphy of core with a depth of about 160 m in the upper half of U1501C site is studied.The core lithology of this section is mainly clay-rich calcareous mud,most of which contain abundant ultrafine fossils and foraminiferal interbeds.Through the experiments of rock magnetic parameters such as temperature dependence curve??-T?,hysteresis loop?LOOP?,isothermal remanence acquisition curve?IRM?,first-order inversion curve?FORC?and so on,it is proved that the main magnetic-carrying minerals in this section are pseudo-single domain?PSD?grains and low coercivity magnetite.The results of magnetic susceptibility anisotropy?AMS?of core samples in this section show that the minimum axis?k₃ axis?dip angle of 88.10%of all samples?312 blocks?is greater than 50 degrees,which proves that most core samples in this borehole are not affected by the later disturbance.In paleomagnetic experiments,alternating demagnetization is the main method,and some typical samples are selected to be demagnetized by thermal demagnetization.Alternating demagnetization data show that most samples acquire stable characteristic remanence?ChRM?after 20 mT.The result improves the resolution and accuracy of polarity inversion sequence preliminarily established by scientists on board U1501C site.It is identified that the depth of the Brunhes–Matuyama transition boundary?B/M?is 27.60 CSF-A?m?,the Matuyama–Gauss boundary is 61.66 CSF-A?m?,and the Gauss–Gilbert boundary is73.29 CSF-A?m?.As well as the two positive polar sub-periods including the Matuyama anti-polar period are Gilsa and Alduwi to Reunivant polar sub-periods,and Keena anti-polar sub-periods during the Gauss positive polar period.According to the results of magnetostratigraphic test,the sedimentation rate of each period can be inferred.