Magnetic properties of Holocene core ZK9 in the subaqueous Yangtze delta and their mechanisms and implications

We measured the magnetic properties of core ZK9 from the subaqueous Yangtze delta to examine the mechanisms and possibility of using magnetic proxies for the sedimentary environmental evolution controlled by the Holocene sea level. The results showed that low values of most magnetic parameters occurred and pyrite formed in the estuarine sediments during c. 10.5-8.4 cal ka BP, reflecting the predominant early diagenesis in a tidaldominated depositional environment when the sea level rose rapidly. Sharp increases in the values of chi (ARM) (susceptibility of Anhysteresis Remanent Magnetisation), chi (ARM)/chi, chi (ARM)/SIRM (Saturation Isothermal Remnant Magnetisation) and HIRM (Hard Isothermal Remnant Magnetisation) occurred immediately above the transitional zone from estuarine to deltaic system of c. 8.4-5.9 cal ka BP, which imply accumulation of single-domain magnetite and anti-ferromagnetic minerals. These magnetic properties indicate the enhanced freshwater processes at the core site as a result of the relatively stable sea level and the associated delta progradation since c. 5.9 cal ka BP. Therefore, magnetic proxies of chi (ARM) and HIRM are useful to identify the maximum flooding surface. In addition, the intertidal-subtidal sediment of c. 13.2-10.5 cal ka BP had high values of chi, SIRM and HIRM, reflecting the features of a coastal environment. The sharp decline of the above parameters from c. 10.5 cal ka BP indicates possibly an event of rapid sea level rise.