Heterogeneity Of Magnetic Diagenesis Zonation In The Yangtze River Subaqueous Delta And Its Implications For Indicating Erosion And Deposition Pattern

Due to the abundant resources and dense population in deltaic areas,delta erosion has drawn great attention worldwide in the context of declining fluvial sediment discharge.Magnetic diagenesis zonation in sediment column can provide information on the relative age of the core,and therefore indicating the erosion/deposition pattern of the delta indirectly.In this study,four cores from the delta front slope facies?20-30 m water depth?of the Yangtze subaqueous delta were subjected to integrated?OSL?dating and magnetic mineral diagenesis studies.The purpose is to explore the relationship between the magnetic zonation and sediment age,and to provide a new approach for the study of delta erosion.The results are as follows:?1?The sediments are dominated by silt followed by clay,and sand is the least abundant fraction in the study area.OSL results indicate large spatial variability of ages.The cores at shallow water depths are younger?<350 a?than those from deeper sites while the cores in the northern part of the subaqueous delta shows much older?>800 a?ages than the core at the southern site with similar water depth,which is caused by their longer distance relative to the mouth of active sediment discharge distributary.Modern deposits?i.e.,<100 years?occur primarily at water depth shallower than 30 m in the southern part?south of 31.5°N?and around 20 m in the northern part of the subaqueous delta.?2?Delta deposits show large spatial heterogeneity in terms of magnetic properties and geochemical characteristics.The cores located closer to the modern depo-center show higher?values,lower total sulfur?TS?values,positive?³⁴S values as well as lower oxidizable iron(Fe_org),suggesting a lower degree of sulfate reduction.On the contrary,the cores distant from the modern depo-center show lower?values,higher TS,Fe_orgrg values,suggesting that they have experienced a deeper degree of reductive diagenesis.?3?Spatial heterogeneity of magnetic properties among the cores is mainly controlled by the particle size,early diagenesis as well as the age variation.In general,hydrodynamic sorting plays an important role in the variations of magnetic properties of young deposits?<350 a?,while reductive diagenesis is more important in the older sediments?>800 a?.?4?The ferruginous to sulfidic boundary?FSB?,defined according to the vertical variation of?,shows statistically significant linear relationship with the deposital age and thus,can be used as a roughly indicator to sediment age.Sediments in zone A?with high and stable??are mostly younger than 400 a.In zone B?with rapidly decreasing??,they are 400-800 a old,while they are older than 800 a in zone C?with low and stable??.?5?The diagenetic zonation which divided by?value as well as the burial depth of the FSB can reflect the spatial pattern of erosion and deposition in deltaic deposits.Considering the quick,simple features of magnetic measurements in comparison to dating techniques,it has great advantages in high-resolution sampling and analysis,and therefore great potential in the study of delta erosion.