Study Of Sedimentary Characteristics And Environmental Impact Of Heavy Metals Of Nansha Seaarea Based On ²¹⁰Pb

In this study,the contents of ²²⁶Ra and ²¹⁰Pb in six sediment cores from the Nansha Sea were determined by high-purity germanium gamma spectroscopy.The activity range of ²²⁶Ra,²¹⁰Pb and ²¹⁰Pb_exwas（8.81±0.96 to 38.5±0.73）Bq·kg^-1,（17.2±1.87 to 143±3.31）Bq·kg^-1,（6.63±2.25 to 110.15±3.84）Bq·kg^-1,respectively.The trends of ²²⁶Ra and²¹⁰Pb in the six cores decreased with the increase of core depth,but there will be slight difference.The main manifestation is that ²²⁶Ra of some stations has a slight increase in the middle section of the core,and then a continuous decrease,and the ²¹⁰Pb in all cores decreases with the increase of depth,and the attenuation curve is more consistent,and appears obvious gradient changes.Based on the attenuation curve of ²¹⁰Pb and the compaction correction of the sediment core,the average sedimentation rates of NS-1-NS-6 cores were calculated by using a constant initial concentration mode of 0.41(cm·a^-1),0.652(cm·a^-1),0.357(cm·a^-1),0.306(cm·a^-1),0.326(cm·a^-1),0.270(cm·a^-1),respectively.The sedimentary age and overall age of 6 core were obtained.The results show that there are two main reasons affecting the sedimentation rate of cores:the one is direction of ocean currents,the other is offshore distance.At the same time,according to the sediment porosity and the corresponding cumulative mass depth in the cores of the 6 sediments,combined with the measured organic carbon content of the particles in the cores,the organic carbon fluxes of the particles organic carbon in the NS-1～NS-6 cores were calculated 3.26±0.05(mmol C·m^-2·d^-1)、6.91±0.12(mmol C·m^-2·d^-1)、3.46±0.09(mmol C·m^-2·d^-1)、5.19±0.07(mmol C·m^-2·d^-1)、5.47±0.11(mmol C·m^-2·d^-1)、4.47±0.09(mmol C·m^-2·d^-1),respectively.Among them,the offshore distance,the water depth of sampling and the primary production of nearshore phytoplankton and secondary production of zooplankton are the main reasons for the difference in core POC flux.The heavy metal content of two long rock cores in the Nansha Sea was determined by inductively coupled plasma spectrometer.The ratio of the measured concentration of heavy metals to the threshold concentration of each layer segment was calculated;in NS-7,the ratios were As（2.0 to 15.9）、Cr（0.3 to 1.0）、Cd（0.01 to 1.2）、Pb（0.6to 1.1）、Cu（0.5 to 1.2）、Ni（1.1 to 3.5）、Zn（0.9 to 1.7）;respectively,in NS-8,the ratios were As（4.6 to 16.7）、Cr（0.3 to 1.2）、Cd（0.01 to0.8）、Pb（0.7 to 1.1）、Cu（0.6 to 1.2）、Ni（1.2 to 3.9）、Zn（0.7-1.6）,respectively.The results showed that both cores were the most polluted by As,followed by Ni.In terms of the average of the overall toxicity calculated by logarithmic decay,the toxicity of heavy metals in NS-7 is ranked as As>Ni>Zn>Pb>Cd>Cu>Cr;the biotoxicity of heavy metals in NS-8 is ranked as As>Ni>Zn>Cd>Pb>Cu>Cr.The Quotients of Mean Probable Effect Concentrations calculated by the biological adverse effect evaluation method shows that in NS-7 and NS-8 core,the range of the mean probable effect concentration quotient are 0.32 to 0.93 and 0.60 to0.90,respectively.The probability of biological toxic adverse effects in the vicinity of the two cores ranges from 15%to 29%,indicating that the degree of pollution in the sea near the core is serious.