Magnetic Properties Of The Sediment In Yangtze River Subaqueous Delta And Its Influencing Factors

In this study,core A3-1(31.64°N,122.36°E,water depth 16 m)and A6-6(30.80°N,122.81°E,water depth 27 m)from the Yangtze River Subaqueous Delta has been investigated.Rock magnetism were carried on all samples,in combination with analysis of Optical Stimulated Luminescence(OSL)dating,particle size measurement,particle size separation experiment,diffuse reflectance spectroscopic(DRS)and geochemical analyses.Analyzing the magnetic characteristics and geochemical element compositions of the sediments of Core A3-1(1000?1200 a)and A6-6(50?200 a)in the Yangtze River Subaqueous Delta,this paper will discuss the effects of particle size,early diagenesis and sediment provenance change on magnetic characteristics.In order to reveal the composition changes of the delta sediments varies over time.Analysis of sediment composition can provide information on the relative influence of river and marine forcings on delta evolution.The main conclusions are showed as follows:(1)OSL dating results show that the age of A3-1 ranges from 1070 ± 50 a to 1180 ±30 a,which is older than A6-6(49 ± 9 a to 198 ± 21 a).It shows that the two cores were formed in different sedimentary ages,and the surface sedimentary ages are quite different.(2)Particle size analysis show that sediments of A3-1 and A6-6 dominanted by clayey silt.The average particle sizes of A3-1 and A6-6 are 24.02 ?m and 22.22 ?m respectively.The grain size of A3-1 is relatively coarse and the grain size changes sharply in vertical direction.The composition of A6-6 is more uniform,indicating that the sedimentary environment of the A6-6 is more stable.(3)Magnetite dominates magnetic properties of the sediments in core A3-1 and A6-6.But the contribution of antiferromagnetic is also exist.While the content of magnetic minerals in A3-1 is lower,and the grains size are coarser,mainly multi-domain(MD)and pseudo single domain(PSD).In A6-6,the content of magnetic minerals is higher,the grain size is finer,the grain size is mainly pseudo single domain(PSD).The magnetic characteristics of the original samples and the size fraction samples show that the ferromagnetic minerals are mainly concentrated in the sand fraction,the fine magnetic grain and incomplete antiferromagnetic minerals reside primarily in the clay and silt(?16?m)in A3-1 and A6-6.Early diagenesis plays an important role in the vertical variation of magnetic properties of deposits in core A3-1 and A6-6.Specifically,in the older A3-1(1000-1200 a)sediments,the content of magnetic minerals is low,the magnetic particles are coarsened,the S-ratio is reduced,and the redness value(R%)is low,which shows the influence of early diagenesis in the reducing environment.(4)Cores A3-1 and A6-6 have significant differences in the compositions of major and trace elements and grain size is an important factor affecting the elemental contents in the two cores.Core A3-1 has higher content of Na,Zr and lower content of Ca and lower ration of Cr/Th and Co/Th compared to A6-6.Comparing with the sediment of the main tributaries in the Yangtze River catchment and the Yellow River,it is speculated that the sediments A3-1 and A6-6 may have different sediment provenances.The ternary plot of Ca/Al-Na/Al-Ti/Al can be used to distinguish the provenances.The geochemical characteristics of A3-1 are closer to Hanjiang River,and A6-6 is closer to Jialing River and Old Yellow River.Firstly,the contribution of different tributaries change with time.Core A3-1(1000?1200 a)is more affected by the Han River,while the A6-6(50?200 a)has a dominant Jialing River influence.Therefore,the magnetic properties of A6-6 are stronger than those of A3-1,and the content of magnetite is more,and ? decreases with the coarsening of particle size.However,the magnetic properties of A3-1 are weaker,and ?increases with the coarsening of particle size.Alternatively,there exists the impact of southward migration of the Old Yellow River in the past millennium.High content of Ca in Core A6-6 in the past 200 years may reflect significant contribution from the Yellow River.As a result,A6-6 has the high content of hematite,the high value of HIRM,the high R%,and low S-ratio,compared with A3-1.