Magnetic Properties Of Late Cenozoic Sediments In The Yangtze River Delta And Implications For Their Sedimentary Environment, Provenance And Paleoclimate

Closely linking the largest ocean and the highest plateau, the Yangtze River Delta is the most sensitive and complex region, becoming an ideal information carrier for global change. Compared with densely studied areas like the Tibetan Plateau and the Chinese Loess Plateau, there is still a long way to go for the Quaternary research especially for the Cenozoic in East China. As an emerging edge discipline in 20th century 70's, Environment Magnetism is widely applied in many areas for its merits-"cheapness, simplicity, convenience, non-destruction for samples, stable records of magnetism, etc.". Currently in the field of Geography, Environment Magnetism is mainly used in analyses of loess-paleosols, deposits of lakes or seas, oils or gases exploration, but rarely applied in the Quaternary sediments especially the Cenozoic.Core SG7 (deep 322 m), located in Xingang town, Nanhui region, Shanghai, goes directly to the bedrock of Jurassic. With methods of granularity and magnetism, as well as lithological character, paleomagnetism, spore-pollen, foraminiferan, etc., it is going to get to the bottom of magnetic characteristics and mineral types, to discuss sedimentary environments, sources and palaeo-climatology significance since the late Cenozoic.Above 150 m of SG7, we collect samples every other 0.5 m while 1 m below 150 m. The number of granularity samples is 431, general temperature magnetism samples 445, thermo-magnetic and demagnetizing samples 27. Figure out the percentage of coarse particles (>1 mm) and fine grains (<1 mm) by laser granularity meter, then calculate average grain diameter (Mz), median diameter (Md), clay (<4 μ m), silt (4～63 μ m), sand (>63 μ m) and fine sand (63～250 μm). General temperature magnetism parameters include X, Xfd, X_ARM, IRM, F_k, S_k, X_ARM/SIRM and X_ARM/X. Thermo-magnetic & demagnetizing parameters include demagnetizating curve, hysteresis loop and thermomagnetic curve.SG7 can be divided into four sections according to magnetic parameters.X value of the bottom (322-284 m), especial 305-302 m, is the highest because of magnetite and maghemite concentration, and with a high ration of incomplete antiferromagnetic minerals at the same time. It is conferred that the temporal sedimentary environment is a paleoweathered crust - paleosols layer according the hot & moist climate as well as fierce pedogenesis.Though the Pliocene (284-204 m) granularity is very big, mainly composed ofcoarse sands containing gravels, the magnetic susceptibility is extremely low and the incomplete antiferromagnetic minerals does not drop sharply. Ferrousmagnetic minerals, with the form of fine SD caused by hot-moist climate and fierce weathered bedrocks which indicate alluvial and diluvia fan of the temporal sedimentary facies, are still main magnetic carrier according to the room temperature magnetism parameters. And the lack of maghemite indicates rapid accumulation of sediments rushed from around hills or plateau and weak pedogenesis.Entrance into the Pleistocene, the land subsidence sped up, transiting from the early alluvial fan to the mid-late alluvial fan (pigtail shape) - peneplain (enforced meander). The most distinct characteristic is the fierce undulation of magnetic susceptibility which has closed relationships with grain sands. Magnetism parameters indicate that dominating magnetic minerals are still ferrous minerals (magnetite) while antiferromagnetic minerals increase dramatically at the same time. The low X sections mostly distribute in stiff clay layers, which have close connection with the cold temperature, deficient sedimentary sources and shallow waters.Magnetic susceptibility is relatively high although the granularity is fine in the Holocene, indicating the concentration of ferrous magnetic minerals (magnetite). At that time, the warm and moist climate, long-distance sedimentary sources expanded to the upstream of the Three Gorges and the Qinghai-Tibet Plateau, rising sea level and widespread growth of modern delta, brought many drossy magnetic minerals to accumulate in this area, with the result of relatively rising magnetic susceptibility.Attention should be paid to that, different from the Tertiary which is mostly controlled by tectonic movement, the Quaternary's magnetic susceptibility and granularity are more affected by sedimentary environments controlled by climate. And high values of X are always corresponding to coarse granularities while the low ones are opposite. For the reason of sedimentary environment differences between the west and the east in China, many factors should be considered including sources, climate, types of rocks, etc., and we should not simply use magnetic susceptibility as a climatic index.