| In this paper,environmental magnetism is used as a research method to study the sediments of A5-3 and A6-5 boreholes in the underwater delta of the Yangtze Estuary by means of grain size,210 Pb and 137 Cs short-period radioactivity dating,photo luminescence dating and geochemistry.On the basis of a high resolution chronological framework combining photo luminescence with short-period radioactivity dating of 210 Pb and 137 Cs,the magnetic characteristics and their influencing factors of the sediments in the Yangtze Estuary underwater delta are analyzed,and the significance of magnetic parameters to hydrodynamic characteristics and sedimentary environment is discussed.The conclusions are as follows:(1)The short-lived radioactivity dating results of 210 Pb and 137 Cs show that the deposition rate of A5-3 drilling hole is significantly lower than that of A6-5drilling hole in the past 100 years,and it is closer to modern deposition and the deposition rate is faster.The-5 hole was found to have an interruption of the 137 CS signal,indicating a faster deposition rate,which is likely to affect the enrichment of210 Pb and 137 Cs short-lived radioactive elements in sediments,complex hydrodynamic conditions and sedimentary environments in the Yangtze River estuary.The use of 210 Pb and 137 Cs short-lived radiometric dating methods is greatly limited.(2)A systematic chronological analysis of the A5-3 and A6-5 boreholes in the subaqueous delta of the Yangtze River estuary revealed that there are some differences in the results between the photoluminescence and the short-period nuclide dating.The age of the shorter-period nuclides is about 40 years old.It is believed that the main cause of the deviation between the photoluminescence of sediments in the Yangtze River and the short-period nuclide age is related to the incomplete drying of quartz particles in sediments.(3)Comparing the photoluminescence dating study with the photoluminescencedating study in the vicinity,it is found that the 4-11 ?m fine-grained quartz also has an age deviation with a deviation of about 60 years.It can be seen that the incomplete retreat of quartz particles is common in the study of photoluminescence dating.At the same time,the coarser quartz particles can reduce the influence of incomplete sun exposure to a certain extent,but it cannot be eliminated.(4)Particle size composition and diagenesis are the main factors affecting the characteristics of the two borehole sediments in this paper.Among them,the magnetic characteristics of A5-3 borehole sediments are affected by the particle size,and also show obvious effects of reduction diagenesis.The magnetic characteristics of the A6-5 borehole are mainly controlled by particle size,where the magnetic susceptibility is closely related to the deposit of <16 ?m fraction components.In the period of relatively stable hydrodynamics,the magnetic susceptibility can be used as an alternative indicator of particle size.(5)The hydrodynamic characteristics indicated by the granularity can reflect the hydrodynamic environment of the region,and the sharp changes in the granularity parameters can generally indicate the occurrence of floods and other disasters in the region.(6)In the sediments modified by the early reduction and diagenesis,the magnetic minerals,especially the ferrimagnetic minerals,will be dissolved in a large amount,resulting in a large change in the magnetic index.Among them,the magnetic susceptibility and the content of ferrimagnetic minerals are particularly obvious,which will decrease with the increase of depth,so the magnetic susceptibility and the content of ferrimagnetic minerals can indicate the early reduction diagenesis.(7)In the experimental results of sediments,it is found that the TOC value decreases with the increase of depth,and the TS value and C/S value increase with the increase of depth.The combination of the three can better discriminate the occurrence of reduction and diagenesis.A comprehensive indicator of the reduction of diagenesis. |
PDF Full Text Request