Magnetic Properties Of Holocene Sediments In Core MQ From The Yangtze River Delta And Its Implications For Sedimentary Environment Reconstruction

The Yangtze River Delta connects the Eurasian continent and the Pacific Ocean, and therefore the sediments contain information of environmental evolution in the land and the sea. Based on magnetic measurement on a core MQ (32°02’N,120° 13’E) from the paleo-Yangtze incised-valley, together with the analyses of AMS14C, particle size, lithological characteristics, geochemistry, X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and scanning electron microscopy (SEM), this paper investigates the down core variations of magnetic characteristics and its relationships with sediment dynamics and geochemical conditions. The purpose is to provide new data for paleoenvironmental reconstruction of the study area. The main results are as follows:1. The core of MQ can be divided into five units (U1 to U5 from bottom upward) according to lithology variations. By stratigraphic correlation with the neighboring core HQ03, U1 to U5 of core MQ are interpreted to be fluvial (Ul), floodplain (U2), delta front (U3 and U4) and tidal flat facies (U5), respectively.2. Magnetic properties of core MQ are dominated by ferrimagnetic minerals. The grain size of magnetic minerals varies in each unit of MQ, in which coarse pseudo-single domain (PSD) and multi-domain (MD) grains domiante in U1, U3 and U4, while fine superparamagnetic particles (SP) and single domain (SD) in U2 and U5. Demagnetization parameters S-100mT values are lower and HIRM100mT (Hard Isothermal Remanent Magnetization, backfield of 100 mT) values are higher in both U1 and U5, suggesting higher abundance of maghemite. The unmixing of isothermal remanent magnetization (IRM) acquisition curves and diffuse reflectance spectroscopy (DRS) analysis show that hematite and goethite are presented in Ul and U5, but U1 is dominated by hematite, while U5 by goethite.3. The influence of provenance on the variations of magnetic properties is to be minor, considering similar geochemical compositions. Influence of early diagenesis is not obvious. The main factors influencing the magnetic characteristics of MQ are:(1) Particle size. Correlation analysis shows that frequency-dependent susceptibility (xfd), susceptibility of Anhysteretic Remanent Magnetization (xarm), XARM/x ,xARM/SIRM, SIRM/x and HIRM300mT (Hard Isothermal Remanent Magnetization, backfield of 300 mT) are positively correlated with the <4 μm,4～8 μm,8-16 μm,16-32 μm,<16 μm and<32 μm fractions, and negatively with>63μm fraction. By contrast, demagnetization parameters S-ratio (S-100mT and S-300mT) display negatively relationships with the fractions less than 32 μm, and positive with the >63 μm fraction and mean size. Magnetic measurements on the sized fractions suggest that the contribution of the 32-63 μm fraction to the bulk magnetic susceptibility (x) is the largest. HIRM3oomT and S-3oomT of the four sized fractions display no obvious trend from U1 to U3. However, from U4 to U5, there are increasing and decreasing trends of HIRM300mT and S.3oomT respectively, as the particle size fractions become finer and the depth becomes shallower. It reflects that antiferromagnetic minerals are enriched in the fine-grain fractions, and the concentration of antiferromagnetic minerals increases as the depth decreases. In U2, fine-grained SD-SP ferrimagnetic minerals are enriched in the fraction of<16μm.(2) Pedogenesis. Floodplain sediments (U2) was formed ca.9000 to 7000 cal. yr BP, when the relative sea level rose rapidly. Abundant fine-grained magnetite was generated due to aggradational weathering. Tidal flat facies (U5), formed during coastal progradation, contains maghemite, goethite and hematite. It indicates the oscillating redox conditions with a fluctuating water table in tidal flat environment. The oscillating redox condition favors the formation of goethite. Abundant maghemite and hematite were formed due to pedogenesis after land formation at a later stage.4. Correlation analysis between x and particle size suggests that the value of x is positively correlated with sand fraction and mean size in fluvial (U1) and delta front facies (U3 and U4) sediments. It is due to the fact that coarse-grained PSD-MD magnetic minerals dominate in these layers, and the PSD-MD grains are enriched in the sand fraction. Therefore, the value of x partly can be used to indicate the strength of hydrodynamics in the Yangtze River Delta.5. The magnetic properties are extremely sensitive to the changes of the geochemical condition. The vertical changes of magnetic properties in the fraction of <16 μm show that HIRM300mT is higher in U4 and U5 and S300mT display decreasing trend from U4 to U5. The trend of HIRM300mT and S300mT parameters (in the fration of <16 μm) is similar to that of bulk samples, indicating that the increase of ferrimagentic minerals from U4 to U5 are mainly controlled by post-depositional alternations. HIRM300mT and S300mT of bulk samples show increasing and decreasing trend toward, respectively, reflecting the increase of oxidation with decreasing depth. In fluvial facies (U1), antiferromagnetic minerals are dominated by goethite, revealing a humid sedimentary condition. Tidal flat facies (U5) contain abundant maghemite and hematite, which can be regarded as the sign of oxidic environment during the process of land formation.