Magnetostratigraphy Of Borehole NHH01in The Central Southern Yellow Sea Mud

In this paper a high-resolution magnetostratigraphic investigation combined with a study of granularity and systematical rock magnetism was carried on the borehole NHH01recovered from the central southern Yellow Sea mud (CSYSM). The core NHH01which was recovered in128drilling cycles (NHH01-1～128) form the CSYSM, with a water depth of73m and a whole drilling length of125.64m, has an average recovery rate of91%and an actual length of115.10m. The sediments of the core are mainly composed of silt and clay, with some interlayers of fine and medium sand.According to the scanning results of volumetric magnetic susceptibility (MS) of the U-channel samples, the core was divided into13sections, and the values of MS are thought to be controlled by the granularity, sorts and content of magnetic minerals. The results of anisotropy of magnetic susceptibility (AMS) indicate the sedimentary sequence has been unperturbed significantly since deposition. The Day-plot of hysteresis parameters, the curves of MS changes versus temperatures and acquisition for isothermal remanent magnetization (IRM) of tipically discrete samples combined with the curves of S-ratio changes versus depth along the core indicate that the main magnetic minerals have a character of single to pseudo-single domain and low coercivities.Three normal and two reverse long sections of the core were revealed by the results of systematically alternating fielde demagnetization. With the data of AMS14C dating, we concluded the reversal between the depth of4.26m and3.84m may be record of the Gothenburg geomagnetic excursion occurred at the beginning of Holocene. Greigite was identified to be the main magnetic mineral of the first long reverse secti- -on by integrated analysis of rock magnetism and sedimentology, and the directions greigite recorded were not consistent with the geomagnetic directions when the stratigraphy deposited, which made the Matuyama-Brunhes boundary (B/M) that initially determined move down. This is the first time to report the exact existence of greigite and its great effect on the interpretation of magneto stratigraphy in the southern Yellow Sea comprehensively.The Matuyama-Brunhes boundary of NHH01was determined at the depth of68.64m based on the results of paleomagnetic investigation and the correlation between the results of granularity analysis and oxygen isotopic curve, and the age of the bottom is no more than1.069Ma which is the onset for the Jaramillo subchron. Combined with the results of borehole QC2and EY02-2, the depth of Matuyama-Brunhes boundary in the southern Yellow Sea could be preliminarily decided at the depth between about60m and70m.